I have a somewhat complicated user upgrade system. I originally bought the Upgrade Coupons addon but had a couple of errors in implementing it. XenSoluce was very prompt in providing a solution to those. I also needed to extend the operation of the coupons and XenSoluce quickly provided this companion addon - Recurring Subscription - to make this work for me. I really can't say enough good things about both the addons and the support they provided to make this work for complicated conditions.