I agree that it should be easier to buy DEC directly with your payment method of preference. However, some advice I have is to watch some cards whose values you know tends to stay around the same range. When they are at the lower end of that familiar range, purchase them with credits (assuming they are not on cooldown), and then hold them for a couple of hours or a day until they go high enough to cover the 5% market fee. That way you can convert your credits indirectly to DEC!