I'm familiar with Paypal (use it for my sites), but from what you describe it, you're throwing a complex solution at a simple problem. That, and I belive Paypal will have to get some more information from the user directly.

FurNetwork has implimeneted a different solution: Seperate buttons for seperate types of products. Since you have a different levels of memberships, this may be the best low-end solution.

You may want to create a redirection page which has the order ready, and redirects to Paypal to let it gather more information.

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