well, they looked pretty much like potato chips

and some of them were even just in their original potato chips snack bags.
Because I did not try any of those, I can't say I am 100% sure about that.
Potato is not their major source for carbs doesn't necessarily mean Mexicans do not eat potato chips though.

For your other question,I was quite busy right before the trip and forgot to take some cash out from ATM. I have to admit that I am not a seasoned traveler either. Oh well.