If you register your United MileagePlus credit card at united.com/RedCross, you'll get 100 miles. You use your MileagePlus account number to register. This seems to work whether or not you still have the credit card (I canceled mine rather than pay the AF). I say "seems to work" because it said I was registered, but the bonus didn't show up yet. OTOH, my wife still has the card, and when I registered hers, I got the same result.
