It's currently not clear if having an extra round-trip (HTTP API - Initiating a Payment Request - steps 2 and 3) is beneficial. The alternative is to respond with an HTTP 402 and the payment request (step 4) when the GET for the initial resource is performed. This removes the need to do an extra GET at the expense of providing a little more flexibility wrt. the location of the payment request service. For example, having the Location header enables software developers to split media servers from payment processing servers. That said, there are techniques to do this today without the extra layer of indirection.