I am using Payza too, but their rule is that if I deposit money I can't not purchase BTC directly.
Oh? That's odd: to be honest, I've only added funds to my account via Bitcoin so far, but I saw no mention of such a limitation when withdrawing.
Is your Payza account verified? I imagine that must be a prerequisite for withdrawing with Bitcoin.
EDIT: Ah, you're right - I've just spotted it:
Important: It is strictly forbidden to buy Bitcoins using funds directly deposited via credit card.