I beleive CCbill will allow you to do this.

My understanding is that the surfer buys a 'block' of minutes which, depending on the script you are using, is calculated into time portions, once that time has been used, the script wont allow the surfer access to the cams any more.

Im also pretty sure as long as the basic mechanism of buying 'blocks' of time is in place, any processor can handle this

Regards,

Lee