Most US carriers eschewed Google Wallet in favor of ISIS, which has only just arrived after years of waiting. Totally reasonable, right? In an apparent effort to smooth things over, ISIS is offering 20% cash back on purchases, but it takes some setup.
First off, you'll need to have a supported Android device from AT&T, T-Mobile, or Verizon – the device list includes most recent phones with NFC. You'll also need to grab an enhanced ISIS-compatible SIM card from your carrier, which should be free. You'll also have to use an American Express Serve account, which is a pre-paid card. Linking it to ISIS will wave all the maintenance fees associated with Serve as well.
If you got all that worked out, you'll be able to hunt down ISIS retailers and start saving some cash. You'll get 20% off your ISIS purchases with a rebate up to $200. So spend $1000, and you'll get a $200 credit back. (The $200 rebate is a total limit per user, not per purchase.) That's a nice discount, but it's not super-accessible and probably won't earn ISIS much good will. At least you can get some free money.