I can easily see why a simple phone call about assembling a bike would get such a wide range of response. Until the parts and frame are in front of the service writer/mechanic they have no idea what you are really talking about, what parts you really have and what problems they will discover along the process that you didn't either know about or didn't tell them first. Some shops try to hope for the best, some are expecting the worst. Some are so busy during the season that challenging jobs (and it's not the actual mechanical aspects that prove to be the time sink often) will be discouraged.

Relationships with a shop are much like any others. They tend to be a mirror WRT the parties involved. A potential customer come in with lots of stated needs and concerns gets a harder line then one who just asks for the service. (And I'm not discounting the importance of explaining what the shop is quoting for or what they do after the job is done. But requests for step by step descriptions when the shop's busy is not the way to get on the good side. We provide service work not instructions as a base line during our busy season.) Anklework's comment about trust is a good one. Just as in love, trust is a vital foundation for going forward.

Lastly, flat rate estimates are just that. An estimate. Take away the chance for the shop to do their full homework and all you have is an assumption not based on the actual situation. Andy.