Pre-authorized payment processor for WordPress?

Hi everybody!

I’m doing a website for a client, which has lots of custom coding in the backend and some of it involves customers management.

My client needs to charge his customers on a monthly basis, but the fee for each customer varies from client to client and on each month. For example:

Customer 1 gets charged $500 first month, $430 the second, etc
Customer 2 $290 first month, $400 the second, etc
and so on

We need to automatically bill each customer at the end of the month without making him enter his credit card each time.

What I’m looking for is a payment processor where I can pre-authorize the customer credit card and bill them automatically, but don’t want their credit card numbers stored in my backend.

Is it possible to do this with Paypal, authorize.net or any other? I know paypal allows recurrent billing, but I think I can only do that for fixed fees.

If the payment processor has a wordpress plugin then even better.

Thanks in advance!

[No bumping. If it’s that urgent, consider hiring someone.]