Shoppers are Not Employees For Paid Shopping...
(disclaimer, I am not a tax professional, therefore you should verify the accuracy of this information as rules and laws change from country to country)
When you become a paid shopper you don't actually work for a company. You will become an independent contractor. That means that the company is not liable for you. You pay our own taxes, insurance, etc. Just like a real business.
Picture it like this, you are in need of having your lawn mowed or your pool cleaned. You pull out the yellow pages and start calling all the ads that look interesting. After you have talked to a few people you decide on one. After they have completed their service for you, you pay they and they move on.
They don't work for you...This is how it works for people who get paid for shopping. They sign up for a mystery shopping company. Complete the job. Get paid. And take care of their own taxes and expenses. (unless you are required to make a purchase as part of your job, then you will be reimbursed)
At the end of the year you will be issued a 1099 rather than a W-2. This means that you are self-employed. By law a company is required to send you a 1099 if you have earned over $600.
When you get paid for shopping you will not have any taxes withheld from your payment. This is the good news. The bad news is that you will have to take care of all your taxes yourself. You can learn all you need to know directly off the IRS site or you could pay a bookkeeper to take care of this for you.
On a final note, being self-employed enables you to pay yourself first. Meaning that you pay your business expenses before you pay your taxes. This is know as a deduction. If you use a spare room for your office you can deduct it. If you use your car for business you can claim some of its expenses.
I encourage you to learn everything you need to know about paid shopping and about being self employed. (here is where you can find the IRS www.irs.gov) You may be surprised how many tax advantages are in your favor.