billing-automation

# 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.c