I think one important thing was lost in this discussion from the original post.
All income must be reported. That is, if you received any shop payments, you have to report them to the IRS, normally using Schedule C because it allows you to then deduct other expenses, such as 50 cents a mile (if documented), to arrive at the profit/loss of your business. And I believe it's correct that if the profit is over $400, then you owe self-employment tax - which used to be double the rate of Social Security taxes for wage employees, likely still is.
But ANY income must be reported. The $400 is only important after you've reported all the income on the Sched C.
And yes, reimbursements can be ignored as income entirely, or claimed as income and then deducted as outright expenses. Don't class them as meals however, because you'll only get partial credit back. This year, my first as a shopper, I just ignored them.
A key item is keeping track of mileage. This is easy if you do it as you go. Ideally, create a spreadsheet for each company you do work for. Make columns for the Date, Job Descript, Job #, time and other things related to it, then for Pay, Cost and Reimbursement. Include a column for mileage and simply use MapQuest or another program to figure the round trip mileage. You want a separate column for Cost and Reimbursement because sometimes you might order something extra that takes you over the limit - or you might not get paid because you didn't complete the shop properly. Any excess Cost becomes a business expense.
Keeping a mileage log like this satisfies the requirements of the IRS. So does keeping a journal in the car but I find that a lot more trouble. And records like this will make you sail through any audit. You'll love getting the 50 cents/mile off the Sched C too!
Other reasonable business expenses are meals you eat on the road for longer shops/audits. If you buy a camera that's used mainly for shops, it's an expense. Paper supplies, ink cartridges, a hand scanner to use on the road or even a laptop if you do out-of-town assignments. Avoid trying to deduct a home office, it's a red flag for audits and requirements are very strict.
(This is my first tax year doing shops, but I've filed as self-employed for most of the past 30 years.)