If you can find someone who'll either not overcharge you for the parts or better still, fit what you supply. Getting rare now though, they've been bitten by people coming back to complain about faulty parts and blaming the garage.
It's not hard to see why garages aren't keen on that tbh.
Firstly they don't know about the part itself if you supply it.
Quality if not experienced with the maker or is it even really a genuine part now that knock-offs are everywhere...?
Then is something goes wrong, even if you trust the garage did everything right, then there is still the hassle of YOU needing to get a warranty claim through your supplier and getting the replacement part and going back to the garage and paying to refit it.
THEY can charge you twice, but probably will feel a bit bad for you and cut you a deal on the return visit, meaning they won't make much on that visit and still have their bay occupied.
Nobody like one job coming back to them more than once, even if it's not because of you not doing it right in the first place.
If they supply the part, they can stop working with manufacturers or suppliers that give them more trouble than they are willing to put up with.
That being said, I'd prefer to pay €5/hr more for labour, providing I get a competitive price on the parts.