If you have a business OPEN card, you can get even more back with the HP offer. I bought a $250 gift card, got the $50 offer statement credit and also $12.50 (5%) from the OPEN benefits. You could also get 2 MR per dollar instead of 5% if it's a Membership Rewards card. Make sure to check that your benefit is set the way you want.

Also, Mr. Rebates doesn't specifically exclude gift cards from their portal language for HP, and it shows up that I will be getting $10 back from there as well. Those sometimes get adjusted when buying gift cards, though, and I'm not banking on that coming through.