Initial commit

2025-11-29 17:57:01 +08:00
commit 46c826cfb9
12 changed files with 4898 additions and 0 deletions
--- a/skills/billing-automation/SKILL.md
+++ b/skills/billing-automation/SKILL.md
@@ -0,0 +1,559 @@
+---
+name: billing-automation
+description: Build automated billing systems for recurring payments, invoicing, subscription lifecycle, and dunning management. Use when implementing subscription billing, automating invoicing, or managing recurring payment systems.
+---
+
+# Billing Automation
+
+Master automated billing systems including recurring billing, invoice generation, dunning management, proration, and tax calculation.
+
+## When to Use This Skill
+
+- Implementing SaaS subscription billing
+- Automating invoice generation and delivery
+- Managing failed payment recovery (dunning)
+- Calculating prorated charges for plan changes
+- Handling sales tax, VAT, and GST
+- Processing usage-based billing
+- Managing billing cycles and renewals
+
+## Core Concepts
+
+### 1. Billing Cycles
+**Common Intervals:**
+- Monthly (most common for SaaS)
+- Annual (discounted long-term)
+- Quarterly
+- Weekly
+- Custom (usage-based, per-seat)
+
+### 2. Subscription States
+```
+trial → active → past_due → canceled
+              → paused → resumed
+```
+
+### 3. Dunning Management
+Automated process to recover failed payments through:
+- Retry schedules
+- Customer notifications
+- Grace periods
+- Account restrictions
+
+### 4. Proration
+Adjusting charges when:
+- Upgrading/downgrading mid-cycle
+- Adding/removing seats
+- Changing billing frequency
+
+## Quick Start
+
+```python
+from billing import BillingEngine, Subscription
+
+# Initialize billing engine
+billing = BillingEngine()
+
+# Create subscription
+subscription = billing.create_subscription(
+    customer_id="cus_123",
+    plan_id="plan_pro_monthly",
+    billing_cycle_anchor=datetime.now(),
+    trial_days=14
+)
+
+# Process billing cycle
+billing.process_billing_cycle(subscription.id)
+```
+
+## Subscription Lifecycle Management
+
+```python
+from datetime import datetime, timedelta
+from enum import Enum
+
+class SubscriptionStatus(Enum):
+    TRIAL = "trial"
+    ACTIVE = "active"
+    PAST_DUE = "past_due"
+    CANCELED = "canceled"
+    PAUSED = "paused"
+
+class Subscription:
+    def __init__(self, customer_id, plan, billing_cycle_day=None):
+        self.id = generate_id()
+        self.customer_id = customer_id
+        self.plan = plan
+        self.status = SubscriptionStatus.TRIAL
+        self.current_period_start = datetime.now()
+        self.current_period_end = self.current_period_start + timedelta(days=plan.trial_days or 30)
+        self.billing_cycle_day = billing_cycle_day or self.current_period_start.day
+        self.trial_end = datetime.now() + timedelta(days=plan.trial_days) if plan.trial_days else None
+
+    def start_trial(self, trial_days):
+        """Start trial period."""
+        self.status = SubscriptionStatus.TRIAL
+        self.trial_end = datetime.now() + timedelta(days=trial_days)
+        self.current_period_end = self.trial_end
+
+    def activate(self):
+        """Activate subscription after trial or immediately."""
+        self.status = SubscriptionStatus.ACTIVE
+        self.current_period_start = datetime.now()
+        self.current_period_end = self.calculate_next_billing_date()
+
+    def mark_past_due(self):
+        """Mark subscription as past due after failed payment."""
+        self.status = SubscriptionStatus.PAST_DUE
+        # Trigger dunning workflow
+
+    def cancel(self, at_period_end=True):
+        """Cancel subscription."""
+        if at_period_end:
+            self.cancel_at_period_end = True
+            # Will cancel when current period ends
+        else:
+            self.status = SubscriptionStatus.CANCELED
+            self.canceled_at = datetime.now()
+
+    def calculate_next_billing_date(self):
+        """Calculate next billing date based on interval."""
+        if self.plan.interval == 'month':
+            return self.current_period_start + timedelta(days=30)
+        elif self.plan.interval == 'year':
+            return self.current_period_start + timedelta(days=365)
+        elif self.plan.interval == 'week':
+            return self.current_period_start + timedelta(days=7)
+```
+
+## Billing Cycle Processing
+
+```python
+class BillingEngine:
+    def process_billing_cycle(self, subscription_id):
+        """Process billing for a subscription."""
+        subscription = self.get_subscription(subscription_id)
+
+        # Check if billing is due
+        if datetime.now() < subscription.current_period_end:
+            return
+
+        # Generate invoice
+        invoice = self.generate_invoice(subscription)
+
+        # Attempt payment
+        payment_result = self.charge_customer(
+            subscription.customer_id,
+            invoice.total
+        )
+
+        if payment_result.success:
+            # Payment successful
+            invoice.mark_paid()
+            subscription.advance_billing_period()
+            self.send_invoice(invoice)
+        else:
+            # Payment failed
+            subscription.mark_past_due()
+            self.start_dunning_process(subscription, invoice)
+
+    def generate_invoice(self, subscription):
+        """Generate invoice for billing period."""
+        invoice = Invoice(
+            customer_id=subscription.customer_id,
+            subscription_id=subscription.id,
+            period_start=subscription.current_period_start,
+            period_end=subscription.current_period_end
+        )
+
+        # Add subscription line item
+        invoice.add_line_item(
+            description=subscription.plan.name,
+            amount=subscription.plan.amount,
+            quantity=subscription.quantity or 1
+        )
+
+        # Add usage-based charges if applicable
+        if subscription.has_usage_billing:
+            usage_charges = self.calculate_usage_charges(subscription)
+            invoice.add_line_item(
+                description="Usage charges",
+                amount=usage_charges
+            )
+
+        # Calculate tax
+        tax = self.calculate_tax(invoice.subtotal, subscription.customer)
+        invoice.tax = tax
+
+        invoice.finalize()
+        return invoice
+
+    def charge_customer(self, customer_id, amount):
+        """Charge customer using saved payment method."""
+        customer = self.get_customer(customer_id)
+
+        try:
+            # Charge using payment processor
+            charge = stripe.Charge.create(
+                customer=customer.stripe_id,
+                amount=int(amount * 100),  # Convert to cents
+                currency='usd'
+            )
+
+            return PaymentResult(success=True, transaction_id=charge.id)
+        except stripe.error.CardError as e:
+            return PaymentResult(success=False, error=str(e))
+```
+
+## Dunning Management
+
+```python
+class DunningManager:
+    """Manage failed payment recovery."""
+
+    def __init__(self):
+        self.retry_schedule = [
+            {'days': 3, 'email_template': 'payment_failed_first'},
+            {'days': 7, 'email_template': 'payment_failed_reminder'},
+            {'days': 14, 'email_template': 'payment_failed_final'}
+        ]
+
+    def start_dunning_process(self, subscription, invoice):
+        """Start dunning process for failed payment."""
+        dunning_attempt = DunningAttempt(
+            subscription_id=subscription.id,
+            invoice_id=invoice.id,
+            attempt_number=1,
+            next_retry=datetime.now() + timedelta(days=3)
+        )
+
+        # Send initial failure notification
+        self.send_dunning_email(subscription, 'payment_failed_first')
+
+        # Schedule retries
+        self.schedule_retries(dunning_attempt)
+
+    def retry_payment(self, dunning_attempt):
+        """Retry failed payment."""
+        subscription = self.get_subscription(dunning_attempt.subscription_id)
+        invoice = self.get_invoice(dunning_attempt.invoice_id)
+
+        # Attempt payment again
+        result = self.charge_customer(subscription.customer_id, invoice.total)
+
+        if result.success:
+            # Payment succeeded
+            invoice.mark_paid()
+            subscription.status = SubscriptionStatus.ACTIVE
+            self.send_dunning_email(subscription, 'payment_recovered')
+            dunning_attempt.mark_resolved()
+        else:
+            # Still failing
+            dunning_attempt.attempt_number += 1
+
+            if dunning_attempt.attempt_number < len(self.retry_schedule):
+                # Schedule next retry
+                next_retry_config = self.retry_schedule[dunning_attempt.attempt_number]
+                dunning_attempt.next_retry = datetime.now() + timedelta(days=next_retry_config['days'])
+                self.send_dunning_email(subscription, next_retry_config['email_template'])
+            else:
+                # Exhausted retries, cancel subscription
+                subscription.cancel(at_period_end=False)
+                self.send_dunning_email(subscription, 'subscription_canceled')
+
+    def send_dunning_email(self, subscription, template):
+        """Send dunning notification to customer."""
+        customer = self.get_customer(subscription.customer_id)
+
+        email_content = self.render_template(template, {
+            'customer_name': customer.name,
+            'amount_due': subscription.plan.amount,
+            'update_payment_url': f"https://app.example.com/billing"
+        })
+
+        send_email(
+            to=customer.email,
+            subject=email_content['subject'],
+            body=email_content['body']
+        )
+```
+
+## Proration
+
+```python
+class ProrationCalculator:
+    """Calculate prorated charges for plan changes."""
+
+    @staticmethod
+    def calculate_proration(old_plan, new_plan, period_start, period_end, change_date):
+        """Calculate proration for plan change."""
+        # Days in current period
+        total_days = (period_end - period_start).days
+
+        # Days used on old plan
+        days_used = (change_date - period_start).days
+
+        # Days remaining on new plan
+        days_remaining = (period_end - change_date).days
+
+        # Calculate prorated amounts
+        unused_amount = (old_plan.amount / total_days) * days_remaining
+        new_plan_amount = (new_plan.amount / total_days) * days_remaining
+
+        # Net charge/credit
+        proration = new_plan_amount - unused_amount
+
+        return {
+            'old_plan_credit': -unused_amount,
+            'new_plan_charge': new_plan_amount,
+            'net_proration': proration,
+            'days_used': days_used,
+            'days_remaining': days_remaining
+        }
+
+    @staticmethod
+    def calculate_seat_proration(current_seats, new_seats, price_per_seat, period_start, period_end, change_date):
+        """Calculate proration for seat changes."""
+        total_days = (period_end - period_start).days
+        days_remaining = (period_end - change_date).days
+
+        # Additional seats charge
+        additional_seats = new_seats - current_seats
+        prorated_amount = (additional_seats * price_per_seat / total_days) * days_remaining
+
+        return {
+            'additional_seats': additional_seats,
+            'prorated_charge': max(0, prorated_amount),  # No refund for removing seats mid-cycle
+            'effective_date': change_date
+        }
+```
+
+## Tax Calculation
+
+```python
+class TaxCalculator:
+    """Calculate sales tax, VAT, GST."""
+
+    def __init__(self):
+        # Tax rates by region
+        self.tax_rates = {
+            'US_CA': 0.0725,  # California sales tax
+            'US_NY': 0.04,    # New York sales tax
+            'GB': 0.20,       # UK VAT
+            'DE': 0.19,       # Germany VAT
+            'FR': 0.20,       # France VAT
+            'AU': 0.10,       # Australia GST
+        }
+
+    def calculate_tax(self, amount, customer):
+        """Calculate applicable tax."""
+        # Determine tax jurisdiction
+        jurisdiction = self.get_tax_jurisdiction(customer)
+
+        if not jurisdiction:
+            return 0
+
+        # Get tax rate
+        tax_rate = self.tax_rates.get(jurisdiction, 0)
+
+        # Calculate tax
+        tax = amount * tax_rate
+
+        return {
+            'tax_amount': tax,
+            'tax_rate': tax_rate,
+            'jurisdiction': jurisdiction,
+            'tax_type': self.get_tax_type(jurisdiction)
+        }
+
+    def get_tax_jurisdiction(self, customer):
+        """Determine tax jurisdiction based on customer location."""
+        if customer.country == 'US':
+            # US: Tax based on customer state
+            return f"US_{customer.state}"
+        elif customer.country in ['GB', 'DE', 'FR']:
+            # EU: VAT
+            return customer.country
+        elif customer.country == 'AU':
+            # Australia: GST
+            return 'AU'
+        else:
+            return None
+
+    def get_tax_type(self, jurisdiction):
+        """Get type of tax for jurisdiction."""
+        if jurisdiction.startswith('US_'):
+            return 'Sales Tax'
+        elif jurisdiction in ['GB', 'DE', 'FR']:
+            return 'VAT'
+        elif jurisdiction == 'AU':
+            return 'GST'
+        return 'Tax'
+
+    def validate_vat_number(self, vat_number, country):
+        """Validate EU VAT number."""
+        # Use VIES API for validation
+        # Returns True if valid, False otherwise
+        pass
+```
+
+## Invoice Generation
+
+```python
+class Invoice:
+    def __init__(self, customer_id, subscription_id=None):
+        self.id = generate_invoice_number()
+        self.customer_id = customer_id
+        self.subscription_id = subscription_id
+        self.status = 'draft'
+        self.line_items = []
+        self.subtotal = 0
+        self.tax = 0
+        self.total = 0
+        self.created_at = datetime.now()
+
+    def add_line_item(self, description, amount, quantity=1):
+        """Add line item to invoice."""
+        line_item = {
+            'description': description,
+            'unit_amount': amount,
+            'quantity': quantity,
+            'total': amount * quantity
+        }
+        self.line_items.append(line_item)
+        self.subtotal += line_item['total']
+
+    def finalize(self):
+        """Finalize invoice and calculate total."""
+        self.total = self.subtotal + self.tax
+        self.status = 'open'
+        self.finalized_at = datetime.now()
+
+    def mark_paid(self):
+        """Mark invoice as paid."""
+        self.status = 'paid'
+        self.paid_at = datetime.now()
+
+    def to_pdf(self):
+        """Generate PDF invoice."""
+        from reportlab.pdfgen import canvas
+
+        # Generate PDF
+        # Include: company info, customer info, line items, tax, total
+        pass
+
+    def to_html(self):
+        """Generate HTML invoice."""
+        template = """
+        <!DOCTYPE html>
+        <html>
+        <head><title>Invoice #{invoice_number}</title></head>
+        <body>
+            <h1>Invoice #{invoice_number}</h1>
+            <p>Date: {date}</p>
+            <h2>Bill To:</h2>
+            <p>{customer_name}<br>{customer_address}</p>
+            <table>
+                <tr><th>Description</th><th>Quantity</th><th>Amount</th></tr>
+                {line_items}
+            </table>
+            <p>Subtotal: ${subtotal}</p>
+            <p>Tax: ${tax}</p>
+            <h3>Total: ${total}</h3>
+        </body>
+        </html>
+        """
+
+        return template.format(
+            invoice_number=self.id,
+            date=self.created_at.strftime('%Y-%m-%d'),
+            customer_name=self.customer.name,
+            customer_address=self.customer.address,
+            line_items=self.render_line_items(),
+            subtotal=self.subtotal,
+            tax=self.tax,
+            total=self.total
+        )
+```
+
+## Usage-Based Billing
+
+```python
+class UsageBillingEngine:
+    """Track and bill for usage."""
+
+    def track_usage(self, customer_id, metric, quantity):
+        """Track usage event."""
+        UsageRecord.create(
+            customer_id=customer_id,
+            metric=metric,
+            quantity=quantity,
+            timestamp=datetime.now()
+        )
+
+    def calculate_usage_charges(self, subscription, period_start, period_end):
+        """Calculate charges for usage in billing period."""
+        usage_records = UsageRecord.get_for_period(
+            subscription.customer_id,
+            period_start,
+            period_end
+        )
+
+        total_usage = sum(record.quantity for record in usage_records)
+
+        # Tiered pricing
+        if subscription.plan.pricing_model == 'tiered':
+            charge = self.calculate_tiered_pricing(total_usage, subscription.plan.tiers)
+        # Per-unit pricing
+        elif subscription.plan.pricing_model == 'per_unit':
+            charge = total_usage * subscription.plan.unit_price
+        # Volume pricing
+        elif subscription.plan.pricing_model == 'volume':
+            charge = self.calculate_volume_pricing(total_usage, subscription.plan.tiers)
+
+        return charge
+
+    def calculate_tiered_pricing(self, total_usage, tiers):
+        """Calculate cost using tiered pricing."""
+        charge = 0
+        remaining = total_usage
+
+        for tier in sorted(tiers, key=lambda x: x['up_to']):
+            tier_usage = min(remaining, tier['up_to'] - tier['from'])
+            charge += tier_usage * tier['unit_price']
+            remaining -= tier_usage
+
+            if remaining <= 0:
+                break
+
+        return charge
+```
+
+## Resources
+
+- **references/billing-cycles.md**: Billing cycle management
+- **references/dunning-management.md**: Failed payment recovery
+- **references/proration.md**: Prorated charge calculations
+- **references/tax-calculation.md**: Tax/VAT/GST handling
+- **references/invoice-lifecycle.md**: Invoice state management
+- **assets/billing-state-machine.yaml**: Billing workflow
+- **assets/invoice-template.html**: Invoice templates
+- **assets/dunning-policy.yaml**: Dunning configuration
+
+## Best Practices
+
+1. **Automate Everything**: Minimize manual intervention
+2. **Clear Communication**: Notify customers of billing events
+3. **Flexible Retry Logic**: Balance recovery with customer experience
+4. **Accurate Proration**: Fair calculation for plan changes
+5. **Tax Compliance**: Calculate correct tax for jurisdiction
+6. **Audit Trail**: Log all billing events
+7. **Graceful Degradation**: Handle edge cases without breaking
+
+## Common Pitfalls
+
+- **Incorrect Proration**: Not accounting for partial periods
+- **Missing Tax**: Forgetting to add tax to invoices
+- **Aggressive Dunning**: Canceling too quickly
+- **No Notifications**: Not informing customers of failures
+- **Hardcoded Cycles**: Not supporting custom billing dates