As much as no one likes to think about it, the truth is that there are some unscrupulous businessmen (and women) who want to get you to pay without having to do much extra work. Here are some of their tactics to watch out for.
The "Special-Price" Trick: This swindle will have the technician grimly telling you that he has 'discovered' extra repairs that need to be done, but then offering this work at a 'special price' with a smile. He'll tell you not to talk to his boss about it, making you feel lucky and conspiratorial. This will cloud your judgment, when you should be realizing that you don't really need the 'repair' on your garage door in the first place, and that the 'bargain' isn't really all that much of one.
The "Home-Warranty" Trick: They sell you a home-warranty with the installation, promising it will ensure that you never have to pay for repair. This is hardly ever true.
The "safety issue" Trick: some unscrupulous dealers may try to override your protests at a too-high price with claims that there is a safety issue at hand, and it would be immoral for you to try to get out of paying extra fees. When they use language like this and it really doesn't seem plausible that it could actually be a safety issue, you're probably being manipulated.
Now, I'm notsaying these are always going to happen to you, or that every home appliance installer is a con artist, or that you should be rude and demanding to people coming to work on your house.... no. I'm just suggesting that you keep your wits about you, stay skeptical and ask intelligent questions when someone comes calling.