This paper is an international law research on (1) the challenges of customs offenses harmonization in the EU, (2) an approximation of Ukraine’s (or other candidate country’s) legislation with the EU customs acquis. The research aims to (a) clarify the situation in the field of customs offenses, (b) analyse previous harmonization efforts, (c) find common denominators, and (d) synthesize the recommendations on how Ukraine shall act to harmonize with the EU customs acquis in the relevant field. At least two successful cases of harmonization are analysed, and conclusions are highlighted. A particular analysis is provided on the impossibility of abrupt alignment or using any EU country as a role model. The article offers an alternative – a gradual approximation by exchanging best practices. The research also emphasizes the importance of the procedural factors in the harmonization process. The article also defines additional possible sources of best practices such as (1) effectiveness, proportionality, and dissuasiveness through the prism of the Court of Justice of the European Union (CJEU) interpretations and (2) European Court of Human Rights (ECtHR) case law.