Some people will refuse to fit customer supplied parts because they are of unknow supply source and quality (they may even be fake replica parts)
I personally will fit customer supplied parts if they are suitable for the application.
however as mentioned above, if 3 weeks later the part in question falls apart of fails in any other way, the return visit will be charged at full rate plus travel. (if I had supplied it, then i bring a free replacement and fit it for free as any reasonable tradesman should)
the other problem that can happen when customers supply parts is sometimes they are wrong or unsuitable for the job,
e.g.
circuit breakers that are the wrong make to fit the distribution board
light switches that match the decor but have the wrong function (2 way instead of 3 way or intermediate)
lights that are not suitable for the location (unsealed fittings in bath rooms or other steamy places)
if the part being fitted is faulty or DOA (Dead on Arrival)
the issue with problems like listed above is that I will not have purchased the correct ones if you have said that they will be on site waiting.
therefore if it then takes me 2 hours to purchase replacement parts, it will be added to the bill at my standard hourly rate.
alternatively, i sit in the van reading the newspaper until you return with the correct parts but you still have to pay for my time.
Ultimately you may be able to save $35 by buying it yourself but this is at the risk of having to pay an extra $150 if there is a problem with what you bought.
not to mention the fact that you warranty for any work done will not cover replacing any parts that fail.