As a mystery shopper, you are considered self-employed, which means that you are responsible for paying your taxes, including both income tax and self-employment tax. If you expect to owe more than $1,000 in taxes for the year, then you are required to make estimated tax payments quarterly.
If you also hold a full-time job, your tax liability will depend on your total income from all sources. It's possible that your employer withholds enough taxes from your paycheck to cover your total tax liability, in which case you may not need to make estimated tax payments.
To avoid being penalized for underpayment of taxes, it's a good idea to estimate your tax liability for the year and make quarterly payments to the IRS. You can use Form 1040-ES to estimate your tax liability and calculate your estimated tax payments.
Alternatively, you can consult with a tax professional who can provide guidance on your specific situation and help you avoid any potential tax penalties.