There are many choices that shoppers must make and like most anything else, many of those choices have to do with whether you choose to purchase goods or services or not.
Some shoppers find that referral services such as Shadow Shopper saved them a whole lot of time researching companies to work with when they were getting started. Of course Jobslinger.com provides job posts in your area for free and even if the particular jobs posted are taken already, at least you know some of the companies in your area that have work. Msjobboard.com also allows you to see what jobs are available in your area. So the choice is whether you choose to pay to play or not.
Some shoppers find the buying certifications from the MSPA have helped them get jobs, other shoppers feel it made/makes no difference whatsoever. So the choice is whether you choose to pay to play or not.
Some shoppers find that the networking opportunities at conferences are worth every penny of the trip. My guess is that experienced shoppers standing out from the masses of newbie shoppers get to network very nicely, thank you, while the masses of newbies and the less experienced get to be the background noise and help foot the bill for this opportunity. So the choice is whether you choose to pay to play or not.
Some shoppers buy various accounting products or systems because it is easier than spending some time developing something uniquely their own that works for them. Your business is a taxable business and you need to have some systematic and contemporaneous record keeping both for your own evaluation of how you are doing and for any possible discussions in the future with IRS. IRS has no specific requirements about how you must maintain your records, but your records do need to be complete and they do need to be in some form that another person can understand what you are doing. So the choice is whether you buy something you may or may not fully understand or develop something that works for you.
Some shoppers choose to buy air cards for laptops for internet anywhere, portable printers, laptops, smart phones and other gear. I have found that 99.9% of the time I can readily enough do reports from home, use my old printer/scanner and a cheap cell phone that I can buy more minutes for when I have a lot of long distance phone shops to do and thus keep my overhead costs down to a dull roar. So the choice for the shopper is whether they are working for equipment or using equipment to get work done.
What I am saying is that you can make mystery shopping a low cost endeavor or an opportunity to spend a whole lot of money which may or may not actually bring benefits you could not have gotten with little or no expense at all. I personally find the best way to have more money is to spend less.