I have only done a handful of jobs over my long career as a mystery shopper for DSG. I have always been paid on time per their payment terms. If you are not being paid on time per their payment terms then I would stop working for them until they have caught up. It does not matter if they don't have anyone else in the area. If this is truly the case then once they catch up and you wish to do work again for them I would increase my rate as you have leverage. How much more simply depends on what would be fair and how fair your payment for the work has been previously. The fact they still pay by check is astounding. Best of luck to you.
Shopping Western NY, Northeast and Central PA, and parts of Ohio and West Virginia. Have car will travel anywhere if the monies right.