Because you will be getting paid by the mystery shopping companies you will work for, it is necessary for them to have some kind of tax identifier for you. In the United States, this can be your Social Security Number or an Employer Identification Number (EIN). If you make more than a certain amount of money with any particular company, they will be required to send you a 1099 for your income tax return. While an EIN is optional when signing up with most companies, it may actually be preferable to have one. If you can provide an EIN, then you may be putting your identity at less risk. You can apply via the IRS website and obtain an EIN in just minutes. You can apply for an EIN on the IRS site here: http://sa2.www4.irs.gov/modiein/individual/index.jsp