Web System to Handle Membership-Site Adjustable Recurring Billing

Hi,

I'm planning on building a drupal-based webapp with customizable monthly billing plans. In short, there will be something like 4 main packages with different amounts of allowed users, storage space, additional features, etc - all costing different amounts. I also intend on including upgrades and downgrades for each plan to offer additional flexibility - which varies the price.

Problem is, the two leading ecommerce solutions for drupal are Ubercart, and e-Commerce - both which completely suck (at least for this scenario). Being that the main app will be drupal-based, I intended on hosting subdomain copies of it for each client. If that is the case, I can have my main site running something else. So my question is, what sort of (hopefully open-source) systems are out there that can handle and manage this type of billing system (the type of system that the other half a million companies are using, etc).

Thanks