In that case, as I mentioned in my previous message, you could define a new service contract for micropayments (micropayments-gateway) -- paypal will be an implementation of that contract.
It is good practice, when specifying a new contract, to try and make it as general as possible so that it will support future implementations from other micropayments vendors -- there's no point of having a contract unless there is a possibility and reason to support other/alternative/multiple implementations.
I'll leave it to someone else to comment on your original post on how to proceed with the details of a micropayments contract.
