We've been keeping advertising and credits separate because depending on the ad size purchase, the purchase amount can be high.
Our top credit amount is $100 and the vast majority of shop items are much cheaper, so this works well. You can buy $100 worth of credits, then purchase a lot of items, so you won't have to refill often.
Ads purchases can easily be over $100, so it would required multiples credit purchases. Even if we add larger amounts, $200 or $500, that would not solve the problem.
The other reason is because it's a lot of work to integrate the credit system into other systems. Aside from the integration, other features would have to be developed on the backend. This update would tough everything, sticky threads, banner ads, bump system, Trade Guardian and upgrades, so it's no small undertaking.
Finally it's a security issue. If someone can purchase $5000 worth of credits then purchase everything with credits for a few months, we'd be in trouble if we were hit by a chargeback. It's safer overall to require lots of small purchases instead of large ones.
Those are the reasons we have never done it. It's something I would eventually want to do because it would actually make things more user-friendly, but due to how big the update would be and the high-risk it presents, we've never done it.
