I think I'd go with Certified RR.  Heard of too many IRS assessments because they 'claim' to receive a paper filing whenever someone gets around to posting it. Friend of mine, (not a client) just got a pretty serious bill because IRS said they didn't receive 2019 1120S paper filing for 6 months after he signed (and mailed) it. 

@taxiowa good to see you're not driving for Uber.