Good point. I wonder, though-- could you make /order/42/payment the only payment resource related to /order/42, so the ID isn't needed? (PUT /order/42/payment)

Good point. I wonder, though-- could you make /order/42/payment the only payment resource related to /order/42, so the ID isn't needed? (PUT /order/42/payment) If the payment doesn't cover the bill, you could have the PUT respond with a link to /order/42/payment/remaining/1, then (if needed) continue with /remaining/2, and so on. That would avoid the danger of a gap between your id GET and the payment PUT.