There are situations where I can do my own diagnosis and know where to purchase parts at deep discount (internet).

Sometimes I don't have the tools or expertise to install parts and sometimes I can't diagnose a problem. I deal with a local independent mechanic for work I can't do and so far he supplies and installs the parts. I would like to supply my own parts for economic reasons whenever possible. I don't want to upset my relationship with my mechanic.

I have two Mercedes cars.

Can anyone here (mechanic or consumer) comment on their experience with this subject?

Thank you.