From Longman Dictionary of Contemporary Englishregistered postˌregistered ˈpost noun [uncountable] British EnglishTCM a way of insuring something that you send by post in case it gets lost or damaged SYN certified mail American English
Examples from the Corpus
registered post• One copy, by registered post to the University Press, a second deposited in the bank vaults for safekeeping.• Her Confidential File etc. has been sent via registered post.From Longman Business Dictionaryregistered postˌregistered ˈpost (also special delivery) [uncountable] a service to insure items sent through the post in case they are lost or damagedSYNCERTIFIED MAIL AmE, REGISTERED MAILBank notes should not be sent unless by registered post. → post 