Cart persistence, checkout orchestration, tax and shipping, and conversion-critical failure handling.
3 questions in E-commerce & Retail.
You have two carts and no rule that is right for every case, so the merge has to be a deliberate product decision rather than whichever write lands last. Prices and stock have both moved since the older cart was created, which is the part that turns a session problem into a correctness one.
Persist the order in a pending state before you touch the payment gateway, quote tax and shipping per seller shipment, authorise once behind an idempotency key, and commit the remaining steps through a transactional outbox and a saga whose compensations void the authorisation and release reserved stock.
Sort each dependency by what the loss looks like. Price and payment authorisation must fail closed because the loss is unrecoverable; personalisation degrades silently. Tax and fraud sit between, and the answer is a written policy with a bounded exposure, not a timeout value.