Without proof of mailing.... there's not much to fight with.  I think I've seen some postings on other list-serves that the IRS is using the arrival date instead of the postmark date.  BUT you don't have a postmark date for a sword.

And for that amount, which indicates substantial income.... a cashiers check???  Paying on-line is advisable, but if not - then using certified mail is.    I know, I know... too late for that advise.

Generally, I've found the IRS will accept the handwritten date on a payment check especially around deadline time as they often don't get around to processing all the mail that arrives that day.