The first thing you need to understand is that businesses mark up the price of a product in order to make a profit. This is true of a plumber, a mechanic, a remodeling contractor, or any service & repair related business. The price that any given business may charge for a product has nothing to do with what you can buy it for over the internet.
If you want to test this theory, go have a part replaced on your car at your favorite shop. Then go to a parts store (or the internet) and buy the same part over the counter. It is not uncommon for resale price to be twice that of wholesale. It is how businesses make money, so that they can pay their overhead, pay themselves, and stay in business.
If the business has failed to provide you with what you originally agreed to, you can always take it to small claims court. The burden of proof will be on you.
I tell you this not to discourage you, but because this is what I have learned in my own experience.