IMSC is a legitimate business that I would consider one of the few exceptions to the general rule of "Do not pay to mystery shop." If I remember correctly, the $50 membership fee also gets a $50 discount to an IMSC conference. As such it can easily be worth it if you are going to one of their conferences. Personally, I have not attended one but probably will do so one day.
PS: There are scams that circulate with two basic themes. The first is a variation on the old Nigerian Bank account scam that purports to be from a MSP (they even steal names from real MSPs). They say, here is a check for - insert amount here. Cash it and send me some of it and you keep the rest. The second offers a list of mystery shopping companies and/or jobs for a fee. That is a bad idea to pay for because that information is available free. Those scams have people saying never pay to mystery shop, which is a good general rule.
Happily shopping Rhode Island and nearby Massachusetts and Connecticut