If you send certified--that is United States Post Office. If PDS then that is mail delivered by UPS, Fedex, et al. Certified only shows something was mailed and received but does not necessarily show what was inside envelope that was received. If they owe, enclosing a check(and having it clear) shows IRS has received the package.
@chasetax " If they owe, enclosing a check (and having it clear) shows IRS has received the package."
But it only shows that there was a check in the package. Or maybe the check was in another envelope, and the package was empty. But I doubt that the anxiety over the mailing address is due to owing taxes -- there's no doubt a refund involved.
IRS Service Centers have their own ZIP Code, but sometimes the 4-digit extension helps get the mail sorted. The Post Office delivers to the physical IRS paperwork factory. I wouldn't stand in line on the day before April 15-in-July, even wearing a mask, to mail something certified. But I have enough outgoing mail, for various reasons, that I pay a monthly fee for stamps.com and get a tracking number and delivery confirmation on large envelopes and packages.