For a head start on all things MS, head over to the New Mystery Shoppers area of the forums and real all of the items "pinned" to the top of the list there.
As for taxes, almost everyone here simply uses schedule C to report income and expenses for MS. If you owe taxes the first year of a new business, there there will be no penanlty for not having prepaid.
A lot of us use Turbo Tax because it walks us through all of the business-related expenses and self-employment taxes.
Most MSers find that using the IRS mileage expense wrte-off is better than figuring auto depresiation and every penny spent on maintenance. (Also, avoids depreciation recapture if you dispose to the vehicle used for MS in future.)
I am not a tax professional, but have been in businesses of various sorts and filing Schedule C for more than 45 years.
Don't worry; be glad that you two have discovered what can be a great source of side (or even main) earnings.
BTW, don't miss doing reimbursement shops for things that you would otherwise have to pay out of pocket for, like car washes, oil changes, hair care, fuel, carpet cleaning, groceries and USPS charges for mailing packages! Reimbursements ARE NOT taxable income !
Based in MD, near DC
Shopping from the Carolinas to New York
Have video cam; will travel