Information on Customs formalities you can find here.
Diplomatic and consular missions issue certificates enabling the import into the country of household items free of customs duties.
The nationals of the Republic of Serbia who have spent at least two years without interruption working abroad are entitled to a customs certificate. Persons who have worked abroad for up to ten years are exempted from customs duties on household items up to a total value of 20,000 EUR. Persons who have worked abroad for more than ten years are exempted from customs duties on household items without any restriction on their total value.
Staying abroad does not represent a basis for gaining the right to obtain a duty-free import. The certificate for duty-free import is applicable only to the import of household items into the Republic of Serbia. It is valid for one year from the date of issue.
To obtain a customs certificate, you need to contact a diplomatic-consular mission of the Republic of Serbia in the country of your residence.