I doubt they were lying, either just not bothered or didn’t know.
I wouldn’t burn any bridges, just ask for a credit, say your research found the shrimp wasn’t suited to your tank, and you understand the sales guy just made a mistake.
I can understand the money you have spend as even on a small tank it soon adds up, but they will have customers with 400g systems buying from there too, their salt purchase alone will probably cover your total outlay, so I probably wouldn’t mention losing your business as that will probably rub the owner up the wrong way, just put it down to learning, you now know the LFS advice is worth as much as someone else’s, no more or no less, until you know it’s worth listening to just use the store and leave the advice alone.
And to be fair, you should have researched what you intend to buy long before going to the LFS, there is a lot to learn on most animals, different options on suitability, turning up at the LFS and expecting the LFS to give you all the facts there and then on a purchase is a mistake. They will give you one opinion, that’s not enough for you to make an informed choice, many times to need to hear many opinions before you can make the choice for yourself.