Today.  1st class stamps are 58 cents I think.

https://www.irs.gov/e-file-providers/modernized-e-file-mef-status

I must have missed the update in ProSeries on e-filing 7004 with a stamp.

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.