RE: If they do not open up the mail and/ or process it on time then that is their problem

By codes and regulations, true.  In practice, it could lead to a mini-disaster.

Allow me to share with you a REAL experience: I paper-filed an extension for an exemption org (EO) in 10/2020 (original due date 11/15).  I did NOT efile because the EO might switch to "someone cheaper" and I didn't want to incur the fee.  The EO later decided to stay with me. 

F990 filed in March 2021.  Late filing penalty (remember for EO, it's $$ per day) assessed within 3 weeks of filing.  Protest letter written with the explanation and copies of the certified mailing and receipt postcard attached.

Four rounds of IRS assessment letters and finally the certified letter for bank lien followed.

Client called and talked to IRS agents (after crazy long wait time, of course) after each letter.  NO positive result.  The assessment was "frozen" for some time but only after the first call. EO also sent response letters - with the original letters attached - after getting each IRS letter.

Finally received a confirmation  earlier this month, stating that IRS kindly removed the penalty.  This is the picture of "their problem". 

I strongly advise against paper filing extensions for 1065, 1120S and 990's, and returns with tax balances.  I don't have fiscal-year 1120S; so, I don't know the answer.  I know this may be irrelevant or TMI.  However, do your best to e-file.   

Another thought, which I don't know the answer because I can pretty much take care of all filing-related thingies on Lacerte.  If the software doesn't allow it, would the IRS website allow extension filing for entities?  Just being George (the curious one, and not the 4Tack one), as IronMan said.

Sorry for the long posting, but I have to keep up with my image....