[FEATURE] add validated query parameter to submit payment route

I see why there is a need for this separate function and it's fine for now. Once we separate the api layer from the express routes there won't be a need for this. The name might be a bit misleading though and I would rather name this method `getTransaction` and the other `getTransactionAndRespond` or something along those lines ;)

Also, the documentation should say `respond` or `return` instead of `send` since this method is not responsible for sending the actual response anymore.