Define rebuild. it can only be what two people agree to do.
Does it include remove the throttle shafts and new shaft seals?
What if something breaks or is found broken? Or thread gets buggered getting out a jet that a PO had trashed?
It's a can of worms.
By mail there won't be a final set up synch or "needle tuning" In person does the $50.00 include the on off the bike, New allen head bolts, polishing float bowls, carb bodies, manifold bolts, clamps, covers, new gaskets, new float valves, various O rings, diaphragms, Floats set by height gauge or filled with fuel and checked with clear tubes? Floats pin hole tested?
Have the cam chain and valves been set? What is compression. Which air boxes, air filters, exhaust, are the exhaust port gaskets good? Is the gas tank clean, fuel lines replaced, petcocks checked, new fuel line clamps. Hows the ignition? What altitude is the bike normally operated at. Lots of room for misunderstanding on what a carb "overhaul" includes or can do for a bike especially when done away from the bike. I have been mostly sticking to bikes in my possession on overhauls but outside work happens.