This is a difficult point because I hate having to haggle but depending on what it is I will haggle, this is very hypocritical.
I will haggle over the price of a car or if I am in a shop, buying a new tv, I will haggle over luxury items.
I also search the internet for cheaper prices then go to the shop armed with this information, I explain that, I understand that the local shop can't match the internet price but I ask how close can they get to it!
This is all done with good humour and in person, never on the phone! your approach and character don't come across on the phone but do in person.
The last time this happened to me I asked to see the other quote, it was a bathroom, I had taken the side panel off of the bath it had been leaking and the floor needed to be replaced, the other quote hadn't taken this into account, I explained this to the customer and they said they'd go for the cheaper quote. I wished them all the best and left.
I think for me its the way the conversation/haggle is conducted, in an aggressive manner "your ripping me off" or "Mate, help me out a bit,"
Its easy to compare a tv, I want a Sony 32 CDI, I can get it for £400 incl delivery. You have a place to start.
This job is different: I quote for a glow worm, you quote is for a WB, I quote for plastic pipe, you quote for copper, all most people look at is the bottom figure.... they don't know the difference (or care) whether its a WB or Ariston, they all do the same.........
People begrudge paying for boilers etc, they are not glamourous there shiny new is, "look at my shiny new mercedes everyone, look how well I am doing" not the same if you say to someone "look at my shiny new boiler, heats the water in seconds, the heating is lovely, my radiators are really shiny"
I try not to take this personally or as an attack on me but it is difficult.