I am looking to implement usage based billing. I would prefer to have a user set a minimum threshold that they would have left (prepaid) in their account. So a user may pay $20 and set the threshold to $10.00. When the user's account gets below $10.00, foxycart automatically charges enough to bring back the account to $20.00.
I am not sure how to do this. My understanding that only subscriptions can auto charge an account and since we don't know when or how much to charge I don't know if a subscription will work.
Any guidance is appreciated.