Cart cashback checkout

Booking a cart whose invoice is settled entirely by the agency's agent cashback.

📄️ Book a cart settled in full by agent cashback

For a cart where the applied cashback equals the grand total, so there is nothing to collect: no gateway order and no proof of payment for finance. Reserves the balance and creates the booking as PAID with zero outstanding, in one transaction — an insufficient balance leaves no booking behind. Rejects a cart that still has an amount to pay; use the offline-payment or guest-payment flow for those.