the date mailed is the date paid. consider that the USPS is short-handed and certified mail requires special handling. call then to see if they can give you an update. this is a paper form. the IRS is also short-handed and the envelope will likely sit a while before being processed.
why not use IRS direct pay. you can specify the date it is to be taken out of your account. if you find out early enough that you need to change the amount. you can cancel the original and set up a new one. after completing all steps, you can print out a receipt. I have never trusted the USPS since a payment to the IRS was returned to me for lack of postage. I had gone to the USPS office to turn it in to a postal clerk so you know there was postage on the envelope. after getting it back after the due date I had to go back to the post office and complained. the clerk checked the area where the stamp was affixed and said it appears there was postage affixed. they wrote a letter to the IRS indicating what happened and that the envelope was submitted before the due date. not wanting to go thru this again I signed up and now use IRS direct pay - no envelop for the stamp to lost.
there is another electronic payment that the IRS offers - EFTPS - but it's more involved to set up. you must register and then wait for the IRS to send you a letter with an authorization code which must be entered before 1st use.
https://www.irs.gov/payments
