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)
```

## Detailed patterns and worked examples

Detailed pattern documentation lives in `references/details.md`. Read that file when the navigation tier above is insufficient.