5500-EZ mailed in late, penalized, can amend?

Form 5500-EZ wants to continue mailing in paper forms. Owner says he mailed it in on 7/31/18, it was not extended.

He has received a $250 penalty letter for being late (10 days) and cannot find or remember if he mailed it certified.

Question: He did extend his Federal taxes. Seeing as he cannot show evidence of mailing it in timely, can he amend as filing under extension to remove the tax penalty?

Incidentally I find it odd that if he mailed it in 7/31 as said, that the IRS would mark it 10 days late.

As a reminder, the "mailbox rule" means a real postmark, which may differ from when you put something in a mailbox.  Could that be the diff?

I think replying to the letter with a copy of the Fed 1040 (or 1120?)  extension should do the trick.

update - Plan Sponsor has found receipt of mailing in this paper 5500ez on time. Address confirmed correct. So wonder why they were penalized for being late and wondering if anyone else has encountered this?

I recall something about the IRS having a "grace period" (unofficial) for mailing... something like 5 days.  So if mailed on 7/31/18, as long as the IRS receives it by 8/05/18, it wouildn't be considered "late".  Apparently the IRS received it on 8/10/18... so 10 days late?  Maybe only necessary to show the IRS the proof of mailing.