Pretty much same thing as it is/should be obvious that there is no snack "everyone in Poland (or anywhere else for that matter) likes".
Well, obviously. I think you are reading too much into what I am trying to say. And I definitely don't have a bad idea of may things being "too expensive" in Poland, if you get the impression that I'm being an arrogant American.
I love Tim Tams and Aero bars, but those are not readily available in the USA. I could buy Tim Tams from Australia and have them shipped, but I wouldn't consider that easy or cheap or a "normal" snack. This is what I was going for with this... snacks that Polish people (in general, NOT everyone ever) might really love, but are difficult to procure. Any info from expats regarding things they miss, etc. I think Reese's are a good bet, judging by a couple responses.
But yeah, for a little more detail, I'm not sure if my exchangee is a Polish national, but she is a current citizen. She hasn't been to the USA, at any rate.
Thanks to those that have responded so far!