That's assuming a point and click (or touch interface). What if I want to have people store my service in a directory or just copy and paste or possibly (not likely) memorize the address? Having that extra level of indirection where you need to click someplace to then get directed to the right "payment address" is possible but a hindrance. Imagine for example those 1900 numbers... you call the number and they get paid 1.99 a minute. They can advertise that. Nice and simple. Not that I am suggesting that's what I am doing here.
I am struggling to work out how you are going to know who has paid for your product/service (unless it is just a gambling website similar to SD in which case just say so).
The point here being that if you can't work out who has paid for what then how could you possibly deliver it to them?