Typically the view is that an extension mechanisms allow negotiation of optional features. A different view is to say we want to move from here to this one place, and that the negotiation mechanism allows movement of a functional core to a new base. Negotiation for combinatorial choices does not seem to work but as a migration strategy it does. Need a mechanism for specifying payment systems so that once there is an idea of prices can decide what the system to be used will actually be. Does not attempt to constrain payments system.
Draft allows a payment or receipt to be put into header.
Allows payments to be done over internet in a common framework. We might expect to provide a transaction mechanism on top of HTTP to permit this to be used. We should expect such matters to be handled by payment system rather than http layer, e.g. server allows interogation of server to find out where transaction got. Some lightweight payment systems will not provide such guarantees, your 2 cents may just be lost. Don attempted to restrict draft to just messages and receipts.