Any good CPA should know how to do this. It does not matter that it is mystery shopping. I do understand that if you did not keep good, accurate records, then it makes it more difficult.
IPSOS and Intellishop 1099s
do not include the amounts that were paid to you though Presto. IPSOS will send you a 1099-NEC for both their Shopmetrics and Sassie platforms, but those 1099-NECs don't include what is reported on your 1099-NEC on Presto. The same with Intellishop.
Notably, the Presto 1099-NEC includes any reimbursements you may have received. You'll report the full amount of the Presto 1099-NEC, then deduct the reimbursed expenses in Part V (Other Expenses) of Schedule C. If you receive payment from Presto through PayPal, you'll need to consider that as well. Again, you'll report the full amount of the PayPal 1099-K, then deduct the full amount Presto paid you on a separate line in Part V of Schedule C.
I realize it can be complicated without good records, and you'll need to go back and recreate records now before you file. If the IRS ever decides to audit you, having records will give you evidence to help fight back. The main thing to keep in mind is to report all of your income, even if you didn't get a 1099. Since the IRS has no way to know what part of your income are fees and what part are reimbursements, you'll deduct the reimbursements out in Part V of Schedule C. You can label the line something like, "Reimbursements included on 1099s." Just something brief to let the IRS know what that deduction is. If you have duplicate income reported, for example, IPSOS income from the Sassie platform will come on their 1099-NEC and PayPal's 1099-K, then you will deduct out the duplicate income in Part V of Schedule C as well. Again, you can label the line something like, "Duplicate income reported on PayPal 1099-K."
Once you follow the process for each MSC, then you'll end up with a net income, which is what you are taxed on. Keep in mind, if you have other miscellaneous expenses that aren't separated out already in Part II of Schedule C, then you can add those on a line of Part V as well. For example, if you are like me and bought a new computer to run your business, you may be able to expense the full amount in Part V by taking de minimis safe harbor election. You or your CPA will need to see if you're eligible or if you will have to deprecate the larger capital expenses.
A couple of other notes, Part V of Schedule C is also where you deduct your loss for shops that were rejected by the MSC. Just have the e-mail(s) or other records to show the report was rejected. If you have non-reimbursed and reasonable expenses, you can deduct those on Part V of Schedule C as well. For example, I used to do hundreds of gas station audits in a year. It was often hard to find something that cost less than the reimbursed amount, so my total might be $1.10 in the c-store, but the MSC only reimbursed $1.00. Well, I could take that ten cents deduction as a loss because the purchase was required in order to do the job, the ten cents was not reimbursed, and the amount was reasonable so as not to raise any questions. Last year I had $166 of non-reimbursed expenses.
Regardless of what others will tell you, meals you buy while out on a shopping route are not legally expendable unless you stopped overnight (e.g. multi-day route without going back home) or you had to stop for substantial sleep or rest to properly perform the work on your route. Words like "substantial" are not defined by the IRS and this is why records are handy as you can keep track of when you left home, your time along the route, and when you returned home. In theory you could be gone on a route for 18-hours and working that whole time, and it might be reasonable if you pull over at a rest area and nap for a couple of hours.