For a snack credit (or I think less than a dollar OOP) you can buy 2 slices of bread. I think the peanut butter and jelly are available at no cost or for a very nominal amount (I can't recall; you'll have to ask). I do know that it's a VERY cheap thing to eat.
If you're talking about uncrustables then yes they are sold in the food court at Pop. I believe if you want to use a dining plan credit for them then you'll need to use a CS credit and get the full kids meal.