The best wat to do that would be to get an SSL cert. I'm using hostcharge right now. They have a place to put in your clients credit card info to charge what ever amount you want. All you would have to do is collect the card info on your site.
We're in the process of integrating with Worldpay and we can do it with them.
They require the price and description (and other details) to be passed to them for processing. All of our plans are flexible and could result in hundreds of different combinations of features and prices from any single plan, but we use a single form to process the payments.
If the processor works by passing the details and prices to them (rather than a fixed shopping cart) then you should be able to do it. You might have to get a programmer involved but they will be able to get it to working without much difficulty.
Why not use the TPP-PRO system with paysystems.com? This allows you do dynamically set all the details of the transaction.
You can dynamically pass in the product, price, recurring info, etc... ask them to send you a copy of the manual for the TPP-PRO account type.
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