Hadnuff, I have used the regular outgoing mail, nothing special, and the Death Certificates had arrived on time to whomever. I have found any that I do mail out will be returned by regular mail, too.
But if you need to mail one out to a place where the mail is always getting lost or mixed up, then mail by certified mail with return receipt request.
I ask for their fax number and I fax them. The fax machine prints out proof of transmittal and I still have the original in my hand. Never have a problem with that.
But if you need to mail one out to a place where the mail is always getting lost or mixed up, then mail by certified mail with return receipt request.