Just for the sake of saying it, I'd tend to go along with the idea that a "thank you for your business" sort of thing would be a coupon or something.
I think otherwise, something like that might be considered along the lines of a "lottery" of sorts, and, far as I know, is not legal to do so in the US without extending the odds to people who haven't purchased anything, except in the case where the purchases is solely for the purpose of gambling. (e.g., buying a lottery ticket)
I could be wrong about that, but I often see similar things in very fine print, informing consumers they need not make a purchase in order to qualify.

In any event, fun or not, that's just a weird way of saying thanks... it's like saying, we appreciate you if you buy more, and maybe, if you're lucky, we'll extend our appreciation a bit beyond how we appreciate others.