Both the parts and service departments need to make a profit, but it CANNOT be at the other's expense. If the parts department sells parts at MSRP to the service department to make their profit, then how does the service department maximize their profit?
Lightly commissioned, but experienced service writers can encourage suggestive “add-on” sales of more parts or accessories and thus more billable labor hours. Voila! Both departments make more and the customer gets accessorizing done conveniently with his service stop.
Warning: Discourage pressuring customers and incorrect parts ordering by uninformed service staff. Either or both can bite you back.