[jamess]

There is one and only one such mechanism that exists, reverse-billed SMS. Even then, the fees payable will often amount to some 25% of the total. What are you going to be selling? If its something you are expecting lots of repeat custom, then you could do as all the music vendors do and aggregate bills over some period of time. Or, if that is a bit too risky sounding for you, have customers charge their accounts up with credit first before shopping.