Honestly after researching on google for significant amount of time... people should really just use certified mail or registered mail when using USPS. You may probably have to do this over the counter in a USPS office, in order to get the certificate or hand cancellation that proves the addressee info, recipient address and mailing date.
The certificate / hand cancellation is the key. No matter USPS actually successfully delivers it or not, no matter the tracking information page is updated or get stuck in "in transit to next facility", etc, doesn't matter. If IRS eventually doesn't receive your tax return, just show them your mailing certificate / hand cancellation and you'll have no responsibility over any late filing, that's it.
One gotcha is adding certified mail is WAY more important than using USPS Express or Priority mail to expedite the delivery. In the worst case, you filed by USPS Express and you get the "Proof of Delivery" PDF file provided by USPS, but somehow IRS says they didn't receive it. You're in trouble as well, since Proof of Delivery, or any tracking history, is not an officially admitted proof for timely mailing. Even worst is neither the tracking history nor the Proof of Delivery contains recipient full address, only state / city / zip code. So you can't even prove you mail to the right address to IRS. Yet you spent lots of money for Express.
So I'd suggest just go for certified mail, doesn't matter if you're mailing as 1st class mail or other "faster" delivery options. Due to the "Timely mailing as timely filing" rule by IRS, you're all set. Of course, private delivery service (PDS) like Fedex, UPS, etc is a different story, you can use recipient for proof in that case, or request the staff to give you a written proof of mailing date. But you'll have to figure out the PDS address of your state IRS Treasury, they can't use PO Box or address w/o street name.
