hot spot, each additional minute scenario

Hi, I am wondering if anyone out there has implemented freeradius in a wifi hotspot where you bill a user's credit card for amount for a certain amount of time that they purchase upfront and then for each additional minute that they want to continue, its an additional charge per minute? I am kind of hung up on how the additional minutes can be added for a user in freeradius in conjunction with how the credit card processing would work in this type of scenario. Currently we have it so that once the user hits their expiration in radius, they have to buy more time to keep going. Any ideas would be great. -will