Yes, you can deduct any mileage related to your Mystery shopping - it is only the commuting (to your W-2 job) that you can't include. You don't have a regular place of work for the side gig. It is always somewhere different so all your miles (for mystery shopping) are deductible. . Think of it like this: Your home/admin office and W-2 job location are both ''regular'' places. The places you go to do the mystery shopping are ''temporary'' places. Commuting is between regular places. It is a business expense/mile if the beginning or end destination is a ''temporary'' place. I hope that helps and doesn't further confuse.
