I have done over 50 shops this month for them. Very good to work with. I have talked on the phone with at least 4 different people in their office. While this is my first month doing mystery shops, I have found Ritter's site easily navigated and the reporting simple. The uploads of photos are are easy too.
I was told they pay 30 days after the end of the month. So for an example, everything that is completed by the last day of the month gets paid the end of the next month (i.e. all completed October shops are paid Nov 30). They mail a check and are located in Toledo, Ohio.
Watch the due dates, if you see one consistently offered but no one justing at it, email the scheduler and make an offer to do it for more. An example might be that they have one that is originally $15, offer to do it for $20. I did something similar for a whole bunch of 3rd shift shops and got dounble per shop compared to what origninally was offered.
The other thing is to recognize with any MSC, is know when they are in a crunch to close the month or finish a project. You can identify this by their emails and the frequencies. While they will not always accept giving more money, if you are consistent in doing good work, or have to travel out of your immediate area, you can secure some higher fees.