i did the same exact shop. i did it right and got paid. they probably did use your shop. i've done a couple for STN and found them to be easy to work with and paying promptly (though I wish they paid more). they have asked me a couple of very precise questions about my shop. they were legitimate Q's but very exacting.
i know two major mystery shopping companies who have lied to their clients by altering my reports (where I missed something or could not provide the answers they wanted) so that they could have the project info complete. one instance was ridiculous, where there was no discernible advantage of the company's product over their competitors and the mystery shopping company was screaming at me to pick who is better and why. "They are the same, there is no difference," I explained. So they the MSC literally said, "You have to choose one or the other. Make something up, if necessary."
I try to pride myself on doing everything perfectly (not just mystery shopping, but school/work/exercise, etc.). however, you might want to consider lying and saying you asked for a quote for 500 and the employee gave you a 300 quote -- you told him 500, then he said, just multiple it by 1.666 and that's the amount. then maybe you get paid.
Some MSCs will look for any excuse not to pay you. It's why I try to avoid shops where I have to put up money and get paid later (like food/restaurant). They find 1 flaw and you're out the entire meal. Shopper beware.