GCU 10% off preowned and $10 off coupon don't stack?

First time I used the $10 off coupon on pre-owned games at my local Best Buy and noticed that the extra 10% off wasn't taken out from the price. Clerk said that it will not really stack. I saw that it prompted him for the 10% off, I just didn't see if he hit "yes" or "no".

Has anybody able to stack this?

Anybody?

Yes, they should stack.

Quick question... Did they Pricematch it??? Because if they did that bones you.