I'm in the camp that you get the regular number of bonus miles plus another 250% of those regular miles.

The reason is the wording of this first sentence, "Shop at 1 store and get a 50% bonus on your purchase."

Then later it reads, "Shop at 5 different stores and get a 250% bonus.**"

If you only shopped at 1 store, it stands to reason that you will get the normal miles plus another 50%. If they meant that you'd get strictly just 50%, then it wouldn't be a promotion at all. You'd be getting half the normal miles, and that makes no sense.