Automate Billing and Subscription Management

Covers subscription billing across billing cycle intervals (monthly, annual, quarterly, weekly, and custom usage-based or per-seat), the subscription state machine (trial to active to past_due to canceled, with paused and resumed branches), dunning management (automated failed-payment recovery through retry schedules, customer notifications, grace periods, and account restrictions), and proration for mid-cycle upgrades, downgrades, seat changes, or billing-frequency changes. Ships subscription lifecycle management, billing cycle processing, and a DunningManager implementation with a three-step retry schedule at day 3, 7, and 14 with matching email templates, a start_dunning_process method that fires the first failure notification and schedules retries, and a retry_payment method that re-attempts the charge, marks the invoice paid and reactivates the subscription on success, or advances to the next retry step and email template on continued failure. Covers tax calculation (a TaxCalculator maintaining region-specific rates - US state sales tax like California at 7.25% and New York at 4%, UK/Germany/France VAT at 20%/19%/20%, and Australia GST at 10% - that determines the customer's tax jurisdiction from their country and, for the US, their state, then returns the tax amount, rate, jurisdiction, and tax type), invoice generation, and usage-based billing - a UsageBillingEngine that tracks individual usage events per customer and metric, then calculates period charges under tiered pricing (walking through rate tiers up to their usage ceiling), flat per-unit pricing, or volume pricing applied to total usage. Closes with best practices and common pitfalls.

Use it when implementing subscription billing, managing failed-payment recovery, calculating prorated charges, generating invoices, or billing based on metered usage. Skip it for anything unrelated to billing automation or outside this scope.

Input is a subscription event - a billing cycle running, a failed payment, a plan change mid-cycle, or a usage record; output is the matching billing operation: a generated invoice, a dunning retry scheduled or resolved, a prorated charge, or a calculated usage bill under the subscription's pricing model.

Framework-agnostic billing logic implemented as plain Python classes (DunningManager, UsageBillingEngine) with no dependency on a specific payment processor; designed to sit alongside a payment gateway that performs the actual charge attempt.

Engineers building or maintaining subscription billing who need concrete dunning, proration, and usage-billing logic rather than assembling payment-recovery and pricing-tier calculations from scratch. Also covers automated invoice generation and delivery, and calculating sales tax, VAT, and GST by jurisdiction.