Custom pickup rings and goof rings
There are times when you need different pickups but they just won't work neatly with the holes you already have in your guitar.
A simple example is when you want to mount soapbar P90s in a humbucker slot - you can't avoid there being some ugly routing showing, and the only solution is a ring to cover the gaps.
Gibson's notorious "goof" rings from early 70s LP Deluxe models were a bizarre example of a factory inventing a solution to a problem they had made for themselves. Later versions routed the bodies correctly for the mini-humbuckers mounted in a P90 soapbar cover. But for some the goof rings have become a desirable addition. While I can make these, it is pretty hard to source the correct ivory/cream material, so they are not always available.
Generally pickup rings and goof rings are simple geometric shapes, so even if I don't have exactly what you need, the design work can be very quick. This keeps the costs down to very affordable levels.
The materials and processes also affect the cost. For example, a flat ring made from acrylic from one of my stock designs might be as little as £10. But if it needs bevelled edges, is made from standard pickguard material, or is a new design, expect to pay more.