Commuting is going from your home to your full-time W-2 job or your full-time W-2 job to your home. Any deviation from that route that is for your Mystery Shopping side gig are deductible business miles. If you are driving for your side gig, it is deductible on Schedule C. You just need to make sure you do not include the commuting miles (back and forth to your W-2 job) or personal miles (anything not commuting or side gig related) in your total miles driven number. If you take a detour (to do mystery shopping work) during your normal commute, those miles are deductible as they are for your Schedule C work and not your W-2 job.
I will include some links below with more information.
https://www.irs.gov/pub/irs-pdf/p463.pdf#page=13 IRS map (scroll up 1 page after clicking the link)
https://ttlc.intuit.com/questions/1901578-can-i-deduct-mileage
https://ttlc.intuit.com/questions/3636848-where-do-i-enter-my-self-employment-business-expenses-like...
