I'm just going to say this from many many years of experience. Do not trust mileage tracking apps, I have learned my lesson and lots of times they are wrong. The best way to keep track of your mileage is to write it down off of your odometer.
I created a Sheets file (Excel) in Google and I input the basic information at home such as mystery shopping company, name of store, address of store, mileage at the start, mileage at each location as well as pay, total miles driven, tolls and reimbursements. It makes it substantially easier to deal with your taxes if you know your exact amount of mileage that you have driven and if you're audited, you have proof of your mileage at every location you stopped at, plus it's backed up to Google automatically and if you choose to cancel a mileage app or it crashes you don't lose all of your data. All I do is open that sheet at each location from my phone and type in my mileage.