Adding an additional subscription service to pro-sites

Hi, pro-sites works well in delivering incremental themes/plugins/disk space so you can create a basic and premium level.

I want to offer an optional enhanced 'feature' that applies to all levels and is in effect another subscription, that isn't tied to the site itself.

Let me give theoretical example, maybe at all levels I only offer e-mail support but if you pay an extra $5/month on any level you get telephone support.

I can't think of a way to capture this subscription at the same time as the pro-site subscription (bear in mind they may only want telephone support for 1 month, but the website for 1 year plus, so the subscriptions may be cancelled at different times)

I can thing I could set up a marketpress shop and 'sell' the subscription there, or maybe better set up a 'membership' site and sell the subscription there (an then in this example I could publish the support telephone number to members)

But both of these would create a disjointed user experience, having to go to different places to manage different subscriptions.

Any bright spark got any ideas? What about WHMCS, doe that do anything like this, I don't know I have never used it?

(please don't get hung up on the business model I used above about telephone support, that is purely an example to illustrate an add-on that may not be hierachical)