Our flat rate amounts are a starting point for our estimates. Sometimes the actual cost is less, sometimes more. Until we see the situation in front of us we are only guessing at any number of multiple choice possibilities. (can't count on the number of times that we find out the provided parts are the wrong spec). (In my cynical moments I wish I could say "if everything is right and fits I'll charge X amount but if not then I get 2X).

A related example is when we are asked about assembling a boxed and shipped bike. The customer usually doesn't initially volunteer the additional accessories like fenders, lighting systems or that they just got back from a 3000 mile tour and the bike has many worn out parts. When we say our usual assembly is X amount they won't want to hear that it's actually going to be 2X given what's really delivered to the shop by FedEx.

So take the bike in a bag down to the LBS and show them everything. Only then will you have a best idea of the assembly cost. Andy.