I'd like to automatically charge my members a variable amount of money based on the services they use on my website. They would accumulate a balance owing, and then every week they would be charged for that amount. Is there a way I can do this without having to store their credit card information in my database?
(Similar to my last question, but I just realized I don't really want to go through the headaches and liability issues of having to safely store CC info)
To accomplish this you'll need to create your own subscription system that manages the "when" of the subscription as well as the "how much". You then can use the API of your chosen payment processor to process the transaction.
An example of this would be to use the Authorize.Net Customer Information Manager (CIM) API to store customer billing information, including their credit card information, and then have a cron job that periodically polls the database and, when a user meets the criteria for a payment, calculate the proper amount and then use the CIM API to process their credit card. The integration is seamless so it will look as professional as you can make it.