99% of the time, I use priority. People often think certified is better because it requires signature, but you have to add tracking separately, which some people miss, and a document or instrument is not considered servered or delivered unless actually recieved when signature is required, unless the party who mails it can prove that the receiving party knew the content of the envelope and refused to accept it. Priority avoids this, but has tracking, so you have a bit more than your word re mailing.