The work time for the tech, of course. The markup (IMO) covers the risk (wrong part, bad part causing warranty work that they have to comp, stock shrinkage/obsolescence, cash cost of holding stock items [rather than charging you two truck rolls], etc.), the time/effort to find, order, receive, inspect, pay the invoice, etc. most of which is not done by the tech or is not billed by the tech as wrenching time.