Every person who buys something in a shop is a customer.
Item is a fancy word for "thing".
If you are not happy with something in the shop (an item or the service), you can make a customer complaint.
to handle/control a situation
to work properly
If something functions the way it is supposed to, it is working properly.
to be faulty
This is the opposite of working properly!
The amount of money that is returned to you is your refund.
Instead of giving you your money back, the shop might exchange the product for another one.
The people who work in a shop (or restaurant) are the staff.
If the shop's staff help you with your complaint, they offer a good service.