Discover is offering bonus cash back in the fourth quarter for grocery store (and restaurant and movie) purchases. The downside is that they cap this at $400 worth of purchases.

Now, I’m not anal enough to keep track of purchases and switch from one Discover to the next when I’ve reached the cap. So I purchased a $400 gift card at my favorite Albertsons. Next month I will purchase another $400 GC using my other Discover card and in December my wife will use her Discover to buy a $400 Albertsons gift card.

Bingo – maximum cash back with zero under use and zero over use. And by buying the gift cards the day after my statement cuts, maximum time before payment (although I’m going to pay in advance of cut date).

Albertsons also sells other gift cards, so we might be buying our Christmas list gift cards there too.

Next time I hear someone complain about high interest rates on credit cards in the U.S., I’ll be sure to pass this story along to them. Wal-Mart de Mexico, which is known as Walmex, plans to ramp up its credit-card business in Mexico by the end of the year, according to Bloomberg sources. All told, the bank plans to issue more than 100,000 cards. The cards will have annual percentage rates that range from 59% to 75%. Ouch. Amazing interest rates, no? Consider this: that’s down from 90%!

American Express is very hip to this game, by the way. Paypal purchases are always scrutinized by American Express. They’re scrutinized because it’s so easy to do what I’ve just described. Small purchases won’t likely gain anyone’s attention at American Express, but you can bet your bottom dollar that big purchases won’t go unnoticed.

American Express, once it has flagged your account, wants to know two things: one, are you essentially doing a cash advance or balance transfer? And, two, are you broke? Indeed, if you are doing these kinds of sham transactions, you’re likely strapped for cash. If you’re strapped for cash, and doing these kinds of deals, then you’re likely a high-risk customer to American Express. If you’re a high risk to American Express, it’s going to reduce your limit substantially or cancel your card altogether.

If you’re going to do these kinds of Paypal transactions, don’t use American Express to do it. They’re on top of the game when it comes to this stuff. Better still, just don’t do it at all. Indeed, this is not the credit environment in which to test the cards companies’ computer systems.