I'm not really sure who would you try to address with this sort of prepay?

Would he get free 7 day trial if he used the pin code?

or are you trying to get those people that don't have credit cards to buy with money. Only I can think of are young teenagers that don't own their own credit cards and extremly low percentage of other people?

I'm not too sure why you want to do a step backwards as opose to going forward?

If anything this industry should start accepting sms billing. It's the easiest, much easier than credit card signup. Fastest. And so frekain common that I have no idea why this sort of payment is not becoming more and more popular among paysites.

You just need to write a code in SMS for example "Condom Site 3" send to certain number and you are instantly billed, and instantly receive reply with your passcode.