I worked at a computer repair shop as a powerless underling. I have seen some people get screeeeeeeeeewed. I'll put out a cautionary tale for the OP, not because I necessarily think he got screwed, but to illustrate how important it is to educate yourself about technology before you put money down for it:
The computer shop I was working at did some shady things. Among them was selling used computers at obscene prices. One day, the most computer-ignorant man I have ever seen in my life walked into the store with his new Wifi USB dongle and his Pentium 4 2Ghz (yes seriously, in 2016) machine and asked us if we could connect his WIFI for him. He lived about 5 miles away and thought we could connect to his wifi from the shop.
Seeing an opportunity to put the screws to somebody, my boss offered him "an upgraded machine at a great price". He was charged $180 for the following
Pentium 4 3Ghz (socket 478)
4GB of RAM
120GB HDD
Some garbage video card
(legal but stolen) Win 7
I estimate that this computer was worth maybe $15 on a good day. What makes this extra funny is that when that machine came in as trade, my manager offered it to me for $20 with a smirk on his face and I turned him down because I thought the price was a rip off. I told him he'd have to pay me $20 to take it. To a dumpster.
What's really funny is that for $225, they were also selling Core 2 8XXX machines with 4GB of RAM and 250GB HDDs, which, again, is a ripoff. They could have sold him one of those which would at least have been passable but they saw a chance to unload an astronomically obsolete hunk of crap on him.
This is what happens when you don't know what you're doing.
Having said all that I think suggesting a 550W (you probably got a junker PSU too) PSU for a machine that is running integrated graphics is quite hilarious.
Just curious, what exactly is the work that was done including part brand and model names? I'd like to see how deeply you were penetrated by this shop screwing you around, if at all.