I have worked with them as a client (they have a web survey panel and I run surveys in my regular job). So basically I "purchase" the use of the people that agreed to do surveys for them. I wasn't aware they also did mystery shopping, but they are legit for what I have done with them anyway.
USamp is their whole name, yes. They are a huge global marketing research firm but I don't think they do a lot of mystery shopping per se which might be why the different protocols.