We also need to describe what happens at the end of a transaction. It is envisaged that the completed transaction screen will have a pointer to several pages that the user can/should/must visit. For example someone joining may/should/must visit the opt-in/out page for mailing lists.
How do we control where they eventually wind up? where they started the transaction from? the home page? someplace else?
Known missing fields:
How and when in the process do we know someone is a member? Do we believe them if they say they are without checking until after the fact?
How are things different if they are a registered user? Are the different if they have order before?
Most of the answers to the above need to be on a Site User's Guide page.