Adding monthly subscription module to an existing ASP.NET website

We have an existing website that was built using ASP.NET web forms/C#, .NET 3.5, Linq2Sql and SQL Server 2008. We would like to add a subscription model to it. Specifically, we are looking for: 1. Functionality in which a user can subscribe from an option of plans (Ex: gold membership, platinum membership etc) 2. The user gets billed on a monthly basis. We are contemplating on the payment provider, but let us know of any suggestions you have. It would either be paypal, plimus, [login to view URL] or other. 3. The user can upgrade/downgrade the plan 4. The user can subscribe for a credit plan (if he/she is not interested in a monthly plan) where the user would pay a one time $x per $y credits. 5. Use cases such as a user upgrade/downgrade/cancel between terms (month) should be handled. We would like this module to use ASP.NET membership and role providers. PLEASE DO NOT APPLY if you haven't implemented something like this before. This would be a best gig for someone who has this implemented for a different site and would like to tweak it for us without having to reinvent the wheel. It's very important that the code has a modular approach and is written with best practices in mind. It SHOULD have a pluggable approach and we should be able to use it on a different website if needed. We would like to stress again that the code should be of highest quality.