What i am saying is the mech might charge him lets say 120 for parts and 300 labor, on top of that GST is applied at the end. This is how businesses operate. In theory you can pay GST twice or five times. Because the business might charge his retail including GST. He doesn't have to pass on what he claims back. Even if retail is say 120 the business still needs to pay GST on what ever the final figure is. No well run business pays its own GST on the final job, they would lose money. Now he may include GST in the price of the parts and make less profit or he may not.