Not sure if this was mentioned before, but just a little PSA regarding the Amex offer:
Google doesn't know the final price until after they ship the merchandise from the merchant. The final price is only estimated. I checked out with a Walmart order of $51 and change, and they ended up only charging $47, falling $3 short of activating the offer. So far Amex has refused to honor it, but I will try again. (The spend can be done over multiple orders, but there is no option to make an order for just $3).
Something to keep in mind.