There should probably be one document describing the payment system and protocol.
Another document should describe the extension to HTTP to support the 402 code (see HttpProtocolExtension). Use [http://www.ietf.org/rfc/rfc2617.txt HTTP Authentication: Basic and Digest Access Authentication] as a model for this document. Before publishing an RFC, shall I submit an Internet Draft?
Links to relevant documents:
[http://en.wikipedia.org/wiki/Internet_Draft Internet Draft] (Wikipedia)
[http://www.ietf.org/ID.html Internet Drafts] (IETF)
[http://www.ietf.org/ietf/1id-guidelines.html Guidelines to Authors of Internet-Drafts]
[http://en.wikipedia.org/wiki/Request_for_Comments Request for Comments] (Wikipedia)
[http://www.ietf.org/IETF-Standards-Process.html IETF Standards Process]
[http://www.ietf.org/rfc/rfc3932.txt Independent Submissions to the RFC Editor]