@physicsogc - what is the issue date of the social security numbers? if after April 17, then yes that is a problem.
ideas:
1) hopefully you paid with a credit card. Call your bank's credit card department ask and ask them for the EXACT time the credit card was used that day at the post office - they will know. Then, go to that same post office and see if they can pull up the receipt of the transaction, given you know the exact time and date the transaction occured. The tracking number is on the receipt. This is a 'long shot' as there are multiple websites that I googled stating the post office doesn't maintain the tracking numbers.
2) is it possible that the paperwork just crossed? meaning you efiled your tax return very shortly after the extension request was mailed in and the IRS hadn't processed the extension request yet? I wonder if the receipt of the extension request is on your transcript somewhere (and that occured after your return was processed?) have you checked your transcript on IRS.gov for an indication that there is an extension on file? if yes, then it's just a matter of contacting the IRS to have them review your tax return again given the dates on the social security cards are prior to the extention date deadline.
maybe others have other ideas....but I would start with the two above.
