ask for money for a service or activity.
charge
A piece of paper you receive when you buy something
receipt
Money given back to you if you return something.
refund
Shopping on the internet.
online shopping
A special offer on a product.
promotion
Helping customers in a polite way.
customer service
A lower price than usual for something.
discount
take something back to a shop and replace it with something else.
exchange
Return an item that you bought online.
send something back
Something that you buy.
purchase