The acronym P.O. stands for "post office," but there are many other words and phrases that can be used as synonyms for this term. Some other options to consider might include "mail center," "postal service," "mailing house," "delivery depot," "mail station," or "postal facility." Each of these terms denotes a location or entity that is involved in the handling and transportation of mail and packages. Depending on the context in which the word is being used, any of these synonyms could be an appropriate alternative to P.O. and help to add variety and interest to your writing.