if you have AmEx AND halo4/live account sync your Amex card to your xbox.
You get $10 for doing so, $25 for beating the game (havent done it yet) and $50 off $100+ at best buy for beating one spartan ops mission.
Dunno if that and 20 off 200 are stackable, but I'd honestly just go for the halo 4 $50 off $100+ deal instead...
that's what I did for my iPad mini
(threw it on a gift card, $100, so I got the credit from AmEx and in case I returned the mini, still got the gift card for a future purchase... already saw the credit appear in my statement days later even though they say it takes up to 8 weeks, love AmEx plus two years warranty)