US Postal Service
How to Prepare Your Outgoing International USPS Mail
All mail being sent internationally should be accompanied by a completed International Charge Slip and must be kept separate from all other mail. International mail that is not kept separate from mail traveling to U.S. destinations could result in having that mail be processed in the wrong classification and will most likely receive incorrect postage.
When shipping overseas, you generally need a customs form for all international mail.* What form you need depends on the service you’re using and the declared value of your mail.
*You do not need a customs form for either First-Class Mail International® items or Priority Mail International Flat Rate™ Envelopes if they weigh less than 16 oz, are no more than 3/4″ thick, and contain only documents.
Complete your customs form using the USPS online tool.
Also, be aware of any restrictions and prohibitions for the country in which you are mailing. Use this link to find the destination country’s prohibitions, restrictions, observations, required customs forms, areas served, etc.
http://pe.usps.com/text/imm/immctry.htm
When addressing your mail piece, the last line of the address should contain only the name of the destination country spelled in English, ALL CAPITAL LETTERS, with no abbreviation. Contact a Mail Operations Shift Leader if you have any questions on how to prepare your outgoing international mail.