If you're going to do your math like that, don't make the mistake of also mentally taking the $20 off the order that you use the coupon on later. That's the biggest problem with these "coupon toward a future order" offers (and one that I've seen GameStop directly prey on with some of their past offer text, very shady) -- people treat them like they're getting $20 off on the game they're ordering and $20 off on a future order (i.e., in the end, like they're getting $40 total off). But they're not, really -- wherever they actually want to consider the discount to be happening, they're only getting $20 off one way or the other in the long run.
Lol I know exactly what you mean. I personally think of it as credit back on the initial purchase rather than a coupon. For instance, if I see a game for $50 on amazon and $55 on gamestop but I have $10 in credit for gamestop, I would still consider Amazon the better deal and get it from there.