The postal service is hit and miss from Cambodia; send anything valuable by courier or from another country. Ensure postcards and letters are franked before they vanish from your sight.
Letters and parcels sent further afield than Asia can take up to two or three weeks to reach their destination. Use a courier to speed things up; EMS has branches at every major post office in the country. DHL and Fed Ex are present in major cities such as Phnom Penh, Siem Reap and Sihanoukville.