Payment processors are good at collecting money. They are not always good at producing the exact invoice a business needs to issue. That difference sounds small until the invoice becomes a legal record. The invoice number, tax treatment, customer address, line items, payment reference, and PDF all need to describe the same business event. If the payment processor cannot express one of those rules, the invoice cannot be treated as a skin on top of the payment provider. That is where we ended up with IGIC, the Canary Islands indirect tax. Stripe still handles the payment lifecycle for us. It charges the customer, tracks subscription state, handles failed payments, and emits the webhook that tells us a payment succeeded. But Stripe does not support the Canary Islands tax model we need for customer-facing invoices. So we draw a hard boundary. Stripe owns payment state. Our billing system owns invoice state. The Iteration Layer Document Generation API turns that invoice state into the PDF customers download.…