I think it should not be possible to buy credits with credits. That does not make sense, especially when the bundle prices are set to a high credits value. For example, when you can buy 10credits for 1€, you can subsequently buy 10credits for 1credit and blow up your credit balance indefinitely.
Also I think it would make sense to block other payment providers inside UNA. This way users could use Paypal etc only for using credits, and credits only inside a UNA platform.