Calculate a discount and the fee together

Given an amount and a discount, returns the fee, the discount taken off, and the final amount the customer pays. Unauthenticated so a checkout page can call it while the customer is still choosing how to pay — it identifies the merchant by merchantId rather than by a key, and reads nothing back that the page did not already send. Send all four fields: a partial body is rejected at the edge before it reaches the calculator. When no discount applies, the response carries the fee alone.