Every part is charged at exactly what it cost and the shop receipt is stapled to your docket. A tap, a cylinder, a breaker, a metre of flexible hose: whatever the shop charged is what you are charged.
The reason is not generosity. A trade that makes its money on parts has a quiet reason to fit the dearer part, or the one it happens to have in the van, and customers can never tell. Taking the margin out of parts removes the argument entirely.
You are also welcome to buy the part yourself and have us fit it. We will tell you honestly before we start whether the one you have chosen is the right one, and we guarantee our own work on it either way.