What's a "one-time subscription payment"? How does that differ from any other one-time payment for something? In other words, can this be used for generalized one-time non-recurring payments, as well as subscriptions?
As to non-subscription payments, we don't support them in the general case, but we're currently in the process of adding support for adding one-time fees to a subscriber's subscription. So for instance if you charge for hosting and want to charge customers for bandwidth overage, you'll be able to do that soon.