Looking at addresses, I've run into both CP or Case Postale, as well as BP, or Boîte Postale. Are the terms interchangeable? Does this vary by country?
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They are not interchangeable in France :
Case postale and boîte postale are synonyms and are both correct. What is similar but improper in that sense is casier postal.
Le Petit Robert lists case postale as a regionalism, so it is probably better to use boîte postale in France, but it is used elsewhere. For example, Canada Post recommends using case postale, but both are accepted.