I payed an estimated payment with my extension but need to file an additional payment before Oct 1, 2025 filing (2024) How?

Hi Turbotax Community (you were really helpful on my last question so here is a "Hail Mary Pass")

I have a complicated tax return due to inheritance and related stock sales (to change the investment objective from my Mother to me!).  Trying for the short story here:  My wife and I are married filing jointly.  Estimated tax payments were made in 2024 for April 15 and Jan 15 (2025 but for 2024 tax year) along with the requested "25%" holding from our social security.  Knowing that I might have significant stock trading income, I filed an extension for our 2024 taxes on April 13 using ACI Payments (this used to be known as Official Payments) and paid a large estimated payment.  For me the neat thing about using ACI payments is that I can file the extension request electronically and also pay by credit card with no requirement to disclose my checking account (credit cards can be cancelled-- bank accounts are more difficult).  Paying this way, the IRS credits my account immediately on the day the payment is submitted to ACI. (Before a host of people point out that I am paying a credit card fee I do not need to: 1) I like the frequent flyer miles, 2) I never give the IRS my bank account (too many frauds have been reported with electronic filing) and 3) I do not link my bank accounts to anything - therefore the IRS free payment site is unacceptable to me 4)It should be obvious by now that I have never filed electronically and never intend to.)  I am a 72 year old Neanderthal!

So: the question is: What is the easiest and fastest way to get one more payment to the IRS in July to stop any interest and payment penalties? (Turbotax currently is thinking it is before April 15 and I have underpaid by about 100$.  I do not want to focus on this now - but a later question I may ask of the community is - how do I make Turbotax recognize that it is July -  left on my own, I would pretend I was filling out an extension on Turbotax Premier without payment and then Turbotax could start charging the interest penalty if it recognized the real date).

At this point my two choices seem to be 1)contact ACI payments.  They seem to only let me do the 4 ES forms (for the 4 specified quarters) and an estimated payment with the extensions request.  I will simply ask them how to make another payment AFTER I have made an initial payment with an extension.  My experience with them is that this is simply "outside of the needs of their usual client base".  I call that "outside of their paygrade".

Or 2) Write a check to the IRS, send it by next day air certified with return receipt (I am a Neanderthal).  This should then force them to agree that the check was received on the day it was written.  I however, have no idea what notation I would put on the check because it is not in any of the ES Payment periods and an additional payment has already been made with my extension.

And finally - it is very likely that I will file a signed copy of our 2024 Tax Return within 2-3 weeks and my final work in Turbotax will be to work through the payment screens in order to get a small refund (with interest payments and any payment penalties stopped by that last "payment" in July).  I do not wish to make a payment with my signed return because of the difficulty in determining the exact interest penalty.  I absolutely do not want the IRS to calculate my interest penalty for me (it is inconvienent and requires me to operate on their schedule, or I may move and the letter may never find me).

The danger in making a final payment in July with a final return being filed shortly thereafter is that the IRS will not properly account for the last payment that I intend to declare on my tax return!

Ideas? Comments?  As a retiree - this is starting to be outside my pay grade (: (Thankfully)

John the retiree