[edit 6/9/14: As Milenomics quickly pointed out on Twitter, BeFrugal is currently offering 4% cash back as well.]
As I explained last week, for the time being I don't have access to American Express gift cards. That doesn't mean I don't know that they're one of the most lucrative tools currently available for driving down the cost of manufactured spend: by clicking through shopping portals to the American Express gift card site, you can earn cash back that more than offsets the purchase and shipping fees associated with the gift cards themselves.
The Barclaycard Arrival RewardsBoost mall gets a lot of attention, whether for its relatively high payouts, the 10% rebate earned when redeeming Arrival miles against travel purchases, or the commissions (other) bloggers earn on successful applications for the card.
Unfortunately, the RewardsBoost portal isn't perfect. First of all, American Express gift cards have disappeared from the portal, at least for the time being. Secondly, if you don't have an Arrival card already, you may not be ready to apply for new credit, or have trouble being approved by Barclaycard for a new account. Finally, as discussed in the comments to this post, if you primarily redeem miles and points for your travel needs, you may simply not have enough paid travel purchases to justify running up large Arrival balances; when redeemed for cash back, an Arrival mile is worth just half a cent.
Meanwhile, if you have a USAA membership, you have access to the MemberShop, another shopping portal operated by the same folks who run RewardsBoost and many of the airline co-branded shopping portals. Unlike those portals, however, the USAA MemberShop offers cold hard cash, deposited directly into your USAA checking account.
It's a little tricky to find the MemberShop: from your USAA home screen, click on "Our Products," then "Retail & Discounts:"
From there, scroll down until you see MemberShop, and click "Shop Now."
The 3% cash back offered by TopCashBack ends in 6 days, and as always it's unclear if that means the rate will be going up, down, staying the same, or disappearing completely. If you have or are eligible for a USAA checking account, it's one more option to maximize your return on American Express gift cards.