Ask HN: SMS Payments - What's the easiest way?

I am selling a product for about $5 and they have to activate it with their phone. What I'd like to do is offer a free demo and then when they send an SMS to our number to activate, it will ask them "Do you agree to pay $5 for this product?" then I can bill their mobile provider. Similar to the ring tones service, or even the Red Cross donation.

How do I do this? I understand the programming, but I don't know of any service that does this.

I thought about asking for their credit card number over SMS but that seems a bit insecure. Thoughts?

Thanks HN, Nick