When it comes to paying the IRS for both your current tax filing year for taxes due (which would be 2020 taxes as of the date of my response) and your estimated tax payments for tax year 2021, I always pay the IRS directly at www.irs.gov/payments and print my receipt. That way, I have proof.
There are three rules to remember when dealing with the IRS.
1) You are guilty until proven innocent.
2) The burden of proof is on the accused (that would be you!) and not the accuser.
3) If it's not in writing, then it did not occur.
