If a shop makes you feel guilty for buying elsewhere because your budget demands it...find another shop.

I have a couple regulars that bought bikes and parts on eBay, etc, and I do sometimes give them the old, "Well, you wouldn't be having this problem if you'd bought from us..." line, but it's just in fun. As AllenG stated, labor is labor and we make no price distinction for straight installations. However, we do offer discounts for installation of accessories when with the purchase of a bike, as well as a discount on the items.