How to integrate with other product sales?

[Hilary]

Here's what I want:

Brand new customer arrives at my site, reads a sales page for a download or service, clicks the 'order' button at the bottom, and is simultaneously added to a vBulletin usergroup - either for a limited time or as a recurring subscription.

Vbulletin's subscriptions provide everything I need, I think - except easy access for new customers. Seems you have to register first, then navigate to your user control panel, find the 'paid subscriptions' part and choose from a drop-down menu.

So... I need a front end that will create those vBulletin subscriptions from a sales page or a shopping cart, making it accessible and user-friendly for new customers.

Many thanks for any suggestions!

(PS - please don't suggest Amember. Not that it isn't great, it is, but it's also what I'm hoping to replace.)