Strategy for modeling a multiple web app subscription system

I am working on a system using php/mysql where I am allowing users to subscribe monthly to various, small browser based web apps. Each app will have different subscription terms and plans. The apps are all currently built and they reside within the same framework.

I am in the modeling phase so I am looking to make this sys开发者_如何学运维tem as flexible as possible wheren the terms from one plan to the next will vary. Any thoughts on how to elegantly model this?

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Rather than building this yourself you could look into using something like Zuora.com. Please note that I haven't used these guys or have any affiliation, I just remember reading something about services like this starting to emerge for web-app publishers needing a simple billing/metering solution.

Of course, you also need to consider which payment gateways you support, but I think that Zuora does that behind the scenes.