Checkout & Payments
One generic checkout, multiple payment rails, reconciled invoices.

Checkout
The storefront uses the generic, cart-backed checkout. The shop registers a checkout source so its lines appear alongside any other vertical's. Selections survive navigation and login.
Payments & invoices
Offer Stripe, PayPal and token payment out of the box; add regional rails as plugins. On capture, the order's line items become invoice lines with per-rate tax, and discounts reconcile to the cent. Payment providers verify their own inbound webhooks.