We have always preached to send ANYTHING (of any significance, even payroll tax returns) to IRS and state agencies by certified mail, with return receipt.

That prevents a lot of finger-pointing and excuses of "we never received it", etc etc.

However.........the certified routine does not ensure receipt.........it only FOR SURE validates timely mailing (filing) and payment being made.

Last summer, client owed 50K of income taxes to IRS.  We efiled the return, so return was filed ok.  

Sent the check by certified mail.

That check was not RECEIVED by IRS for 3 months.

IRS wanted to assess late payment penalties and interest.

We were able to show timely payment, plus the several IRS centers where the certified letter/check was sent, before it ended up being delivered to the address to which it was addressed.........and IRS waived the penalties and interest, which was a no brainer, but it cost me at least 4 hours of dicking with them.

So..........use certified mail whenever it is important to prove timely initiation of the process, but make sure you can prove the route, etc. with ultimate delivery somewhere because you might have to deal with someone at the IRS/state agency who has been bitten by a scammer before, and says "prove it".