I don't think of it as getting the three items for $180. I got three items for $280. At some point in the future, I'll get $100 worth of stuff for free - so in the end I'll get $380 worth of stuff for $280. But I don't have the extra $100 to put in my bank account now, it's just a credit for more stuff, so it's not really $100 off the price of the current items.
It can still be smart to spend a few dollars more, if you have say $260 worth of merchandise ready to ring up, and adding an extra $20 will get you the credit for future purchases.