The pay is hourly and it varies by experience and location. It's above minimum wage and you get paid for your travel and reporting time as well, making it better pay than most MS jobs. As you get better at the job, you realize there is little benefit to working faster, since you are paid hourly. You will spend a lot of time walking around grocery stores, pretending to shop.
Reimbursement for groceries is also very limited, meaning there's no specific reimbursement offered, but often they will require you to purchase items that test the abilities of the checkers, and those would be reimbursed. For obvious reasons, that means many small batches of fruits & veggies. You will spend more on groceries than you earn on the job.
Also remember that as an employee, you have little say in when/where you shop. You offer your time frame of availability and are assigned random stores within that time frame. You may often be shopping at a store far from home, but those are the better shops anyway, since you get travel pay.
In general, if you are a homemaker and like this grocery store, this is a much better shopping opportunity than FF shops. It will add a few thousand a year to the family budget and your fridge will be filled with all sorts of interesting vegetables, rather than burgers with a bite taken out of them. If you do not like to cook or do not have another income source in your household, this job would not be a good fit.
Also keep in mind that this is reported income, and you will get a W-2 for your earnings at the end of the year. If adding $3k a year to your household income will put you in a higher tax bracket, it may end up costing you $5k in taxes to work for them. I learned that the hard way,..