Just be sure to screenshot *everything* and keep good records, and take the time to manage your account to ensure you get the mileage due. Not all vendors are reliable with posting mileage credit. US won't give you the miles till the vendor says so, and Freecause (the company managing the US shopping mall) is pretty much useless in the consumer support area. If you're dealing with vendors like drugstore.com and other long-term mileage mall companies, it does run pretty smoothly, but if there's a new merchant or someone who doesn't get a lot of traffic, you might have to fight for your miles. And if it's one of those bonanza-type offers that seems too good to be true, you might *never* get your miles because the merchants will find any loophole possible to get out of paying up.
I am currently involved in a mess with a vendor who won't pay up and from this thread it seems like there's other FT'ers in the same boat. It has not been a fun ride and it's probably not over anytime soon...
http://www.flyertalk.com/forum/us-ai...t-honored.html
Finally, buy only what you would already be buying OR what you can afford to gamble. If you walk into what you think is a good deal, only to find the vendor won't honor their offer, you may find yourself with no miles and just a credit card bill. Of course some vendors are better than others and if you stick with the reputable ones you have lower risk but I still wouldn't go out and buy things from the shopping mall just for the miles unless I could afford to lose the money.