Web Payments and voucher URIs

I have written a payment server that can use arbitrarily many
authentication methods on the web.

The outcome of that authentication is to return a verified URI.  You could
think of it as a super set of WebID, DID, user addresses and so on.

One thing I'd like to do is have a voucher system.  So the idea with a
voucher is that it has a special code, say you email it to someone, or have
a scratch card or something.

Then when that code is shown the back end is able to let the user spend
whatever balance it is for.  So it's a long the lines of a voucher, a
shared secret or a one time password.

This may be similar to a bearer token, im not sure, as Im not so familiar
with those.

My question in all this is, given that I need a URI that is linked to the
voucher.  Is there something existing I can use.  Or, is there some
sensible standard we can start experimenting with.

The idea I had was to use the URI

*urn:voucher:12345abcd ...*

And if that appears in the request you know. the user can spend the
voucher, and that allows me to build an app.

Any thoughts, ideas or previous work that can be reused here?