If you send a letter or package by certified mail, you send it using a mail service which gives you an official record of the fact that it has been mailed and delivered.
[US]
We recommend that you send your certificates by certified mail.
certified mail in British English
(ˈsɜːtɪˌfaɪd meɪl)
noun
US
first-class mail that requires proof of delivery, for a small fee payable by the sender
certified mail in American English
US
1.
a postal service for recording the mailing and delivery of a piece of first-class mail
2.
mail recorded by this service: it is not insurable