I addition to our main subscription, we offer a largish number of optional subscriptions. In addition, we offer annual, quarterly and monthly versions of our subscriptions. This large number of options can be confusing to customers and I often have customers order the wrong subscriptions. My ideal solution is one where I can: 1. Create an account for a customer 2. Add certain items to the account on behalf of the customer but leave these items unpaid. 3. Invite customer to login and pay for the items I have selected for him and that are "unpaid" in his account. What is the best way have a customer login, the "unpaid" items in his account and pay for them.