also do not let them tell you that a "member of staff has wrongly priced the item and you can't have it at that price" again it is their problem not yours and they must sell at the posted price.
I think here the law differs with the UK, if this is so for PL. In England & Wales, a physical (not online) supermarket or other shop can put the wrong price on and they don't have to sell it at that price.
If an item is priced incorrectly on the shelf, or scans at the wrong price at the till, retailers are under no obligation to honour it, under the Sale of Goods Act. They can offer the item at the correct price or refuse your money and withdraw the product from sale.
This may well be different in Poland, however, as Biedronka refunded my money recently when the shelf price was almost half the price of the price required at the checkout. There was no fuss, they were very nice about it.