I've only been doing mystery shopping for a year, and although I have had some very late payments, I have yet to be cheated out of any payment. I'm signed up with 20+ MSCs and I have about 400 shops so far. I do agree with the OP (Nabilino) that there is no easy way to track all of this stuff. It's all manual, and if you aren't keeping good records as you go, you will end up with a big mess.
To Nabilino's point, it would be very awesome if we could export job reports, payments, etc. from the Sassie platform. The same goes for other MSC platforms - AND PayPal. Some MSCs are great with their PayPal payments and they include the job ID number with the payment. This is helpful when you receive a single PayPal payment for multiple jobs with the same MSC. Most MSC's PayPal payments only have a generic statement like "this if for payment for jobs completed from Feb 1 through Feb 16."
I use one spreadsheet with a separate tab/worksheet for each calendar year. I use it to track mileage, expenses, date of reimbursement, method of reimbursement, etc. My "paid" field changes to green automatically when I enter a paid date into a certain field. All of the info I track in this spreadsheet also made it easy when it came time to report taxes for the past year - that's why I keep all of the calendar year info in one tab/worksheet. It's great for filtering, sorting, etc.
Speaking of sorting - one tip that I read in this forum that has really helped me out, is to include a column on the left hand side of my spreadsheet for the line item number. I enter shops in my spreadsheet in the order that I am approved for them, not the date that I peform them or submit them. This numbering adds another way to resort your spreadsheet if you screw it up by sorting by another column and then want to go back to your original sorting format. (you can have Excel or Google Sheets automatically number them, BTW)
TL;DR: Keep good records as you go, to save a lot of time later. Also, the MSCs could do a better job by providing the option to export payment reports, etc.