A bargain is something which is cheap, a good deal, something which is less expensive than it normally is.
A discount is a sale, a bargain. However a bargain could be in an informal scenario (selling something privately) whereas discounts and sales are for stores only.
A price tag is the little piece of paper which is attached to something and has the price written on it.
A receipt is the paper the seller gives you after you make your purchase. It says what you bought and how much it cost.
A refund is when the store gives your money back to you for something you bought.
When you take something back to the store (return it) they will give you your money back, in many situations.
I hope that helps