I've purchased the Site Membership Pro for a customer - does exactly what they want, perfect! However, the customer also wants to offer products for direct purchase through the web store - including the site subscription option.

From a usability standpoint, I can understand their desire to have all items available from a single purchase point - it's counter-intuitive to send users to various points in the site to make purchases. Even with the Membership feature included on the same page as the Web Store, users are forced to make separate transactions for the subscription-based membership, vs. any other products or services. Forcing multiple purchases makes it less likely that the customer will come back for the additional purchases they would have made otherwise.

Ideally, the web store would be able to detect the presence of Site Membership Pro, and if present, provide the fulfillment option of "subscription", in addition to "physical" & "download", with a conditional dialog to set the terms of the subscription. This will allow the user to make multi-product purchases that include any subscription-based items. 

Well, allrighty then. Can't blame me for trying - thanks for giving the question consideration!