Income taxes are "pay as you go"---so if you expect to owe more than $1000 at tax time, it would be in your best interest to make estimated payments during the year instead of letting it all pile up and having a big tax bill plus an underpayment penalty at tax time.
Since you have self-employment income you owe self-employment tax for Social Security and Medicare as well as ordinary income tax. You are responsible for reporting ALL of your self-employment income whether they issue a 1099 or not, so make sure you are keeping your own careful records of the amounts you get from the mystery shopping job.
You could start making quarterly estimated payments or you could also have more withheld from your paychecks from your W-2 job. That could be done by preparing a new W-4 for the job that withholds tax -- on line 4c of the W-4 you can request to have extra tax withheld from your pay.
https://turbotax.intuit.com/tax-tools/calculators/w4/
https://www.irs.gov/individuals/tax-withholding-estimator
https://www.irs.gov/pub/irs-pdf/fw4.pdf
https://ttlc.intuit.com/questions/1901110-do-i-need-to-make-estimated-tax-payments-to-the-irs
https://turbotax.intuit.com/tax-tools/calculators/self-employed/
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
