Done a lot of either price verifications, and the business verifications. They pay fast, and reliably. Think about how much time you have spent checking other companies, and how long "45 days after the last day of the month your shop took place..." can get.
The $4 and $3 are for each 25 items - and you have the option of checking 50, 75, 100 or even 200 items. I have been happy with these. I have gotten good at finding the items, and have "taken home" $100 or more in a month for buying groceries.
The business verifications - I find these very simple and clear cut. I won't travel for these, but $17 or $19 is just about what I want from a work day....
I want to make enough money to buy a new lap top or tablet every year in my retirement.
If I don't schedule a mystery shop for each day the week, I will allow myself to sit on the couch and drool - not healthy.