I guess I understand where his increased charge for installing your parts comes from. You are giving him new parts, and if there are any problems with those parts, you'll want him to make good on those parts without paying him more money. At least that is what he expects.
If he provides the parts, a parts problem is something he can settle with his supplier and he has less risk.
If it were me, I'd put it together myself instead of paying him to do it. It might take longer and even cost more, but it will be more satisfying. I love the feeling of going down the road at speed knowing I assembled all those pieces.
