From zane@genesis.mcs.com (Sameer)

The second solution I thought of seems like it would work. When I create the return-address block, it can be given some sort of ID-code (again, like with my other idea posted, similar to the ID-code on peices of Digicash in Chaum's scheme) so when the vendor delivers the product, she sends to encrypted block to the remailer, and the remailer forwards the product to me, and stores the ID-code in its database (doing the proper one-way transformation for untraceability) so that further attempts to use the exact same address-block will be noticed and not delivered.

Why not just include an Expire: header in the encrypted block, after which the remailer would just junk any mail sent with that return address?