Finalizes a new eSIM purchase and provisions the eSIM.

Billing Notice: Calling this endpoint represents a binding purchase. Your prepaid partner balance will be immediately deducted by the wholesale cost of the item(s) in the cart.

Idempotency (by cart_key): This endpoint is idempotent. If you retry with the same cart_key after a successful submit (e.g. client timeout or network error), the API detects the existing order and returns 200 OK with the same orderID and purchases. No duplicate eSIM provisioning, wallet debit, or confirmation email is triggered.

Integration tip: On timeout, safely retry POST /orders/submit with the same cart_key. Compare the returned orderID to confirm you received the original order.