Always makes me laugh when pros on here say about building into price cost of going back under g`tee...Do you not have faith in your products .
So all the boilers, pumps, stats, taps, valves, etc, etc that I fit are made by me in my shed (OK the Glow Worm boilers are) are they??
When you fit a 'product', you may be fitting someone else's manufacturing defect, but its your extra time used to change it under guarantee.
When your car is serviced, they give you an invoice, including labour and parts. Garages don't sell you these parts at cost, so why should plumbers?