IF you want to continue working with them, I would give Matt a call or an email after doing some research. I am assuming that the 192 shops done are within the same time period as the 9 questions, and it is the quality of those questions that counts. 9/192 is a 4.6% 'error rate', which is high, so you need to review those 9 incidents.
1) Has he had the same issue twice?
2) Did most of the questions 'cluster' around when you were first starting to do work for them?
You mention that all mistakes were fixed by receipts. What is the pattern there (if any) and what proactive steps can you take to assure him that you are now 'on your game' so they will not occur again?
Think it through and have a civil discussion with him because evidently he needs you if he is sending you out to the hinterlands.
My comments were not personal. We all goof up and sometimes we are called on it and sometimes it is let pass (or not spotted at all). From Matt's viewpoint, you are a 4.6%, which is why it is important to look at where the problems came from. New-to-them shopper is much more likely to have errors than more experienced shopper for them. Errors can be due to poor instructions or carelessness or just plain flubs or unrealistic expectations (like typing a large number of nonsense 28 character combinations of numbers and letters). Once you have done your own evaluation of what the issues have been, then it makes sense to have the conversation with Matt if you want to keep shopping for the company.