or you can avoid sending a check and voucher completely. the IRS has direct pay which will allow you to make an electronic payment using Direct Pay. at the end you get to print a receipt and the IRS even sends an email confirming the withdrawal
https://www.irs.gov/payments
there are things you have to be careful about
specifying what you are making the payment for - 2019 balance due or 2020 estimates
routing number that the 9 digit number at the lower left of your checks and the bank a/c number
if you prefer mailing the check and voucher follow the instructions for the info that you must put on the check.
