Go read on the irs website about self-employment deductions. There have been recent changes in some of the information.
I showed a couple hundred dollars of shopping income last year by only claiming "additional" miles. A lot of my mystery shopping I do while running other errands. I keep track of which miles I drove only because I had shops, and which miles I would have driven any way. (I might plan a day of mystery shopping on the way to a doctor appointment 100 miles away, for instance. I can choose to call those work miles or medical miles or split them between the two activities.)
If you read the small business information on irs.gov you'll have a better idea of how this all works and what options you have.
Time to build a bigger bridge.