If you present $500 on a ticket and did $1000 of shops at the destination, you could reasonably IMHO make a case that the reason for the trip was mystery shopping and that visiting a friend was secondary. A $500 ticket and a $20 mystery shop is not a situation where I would personally try to claim the $500 as a business expense.
Shopping Southeast Pennsylvania, Delaware above the canal, and southwestern NJ since 2008
A more typical situation where claiming the travel expenses while shopping would be as follows:
I drive 225 miles, paying a lot of tolls to a relative's home. Along the way I do a $40 shop. While visiting there I drive an additional 50 miles to take several local shops. But, any other mileage while there is "personal." I drive home, doing a couple of $20 shops along the way. Fuel and tolls = $60. Shop fees = $200. I do not deduct anything for taking my relative out for dinner (not a shop) because they saved me paying for a hotel.
Now let's suppose that I made the same trip to get a chance to tour NYC. My 2 hotel nights and meals while there cost $600.00. I performed the same shops for fees of $200. So, I have $660 in expenses for $200 in fees. While it is not unreasonable to lose money on some routes, out of an abundance of caution I will report only $300 in expenses for that trip. The IRS is NOT looking for any trip that lost money; they ARE looking to see if, over the reporting YEAR you consistently had a goal of making a profit BY YEAR'S END.
Based in MD, near DC
Shopping from the Carolinas to New York
Have video cam; will travel