There are many synonyms for the phrase "slip by the customs." This phrase typically refers to avoiding customs officials or bypassing customs procedures. Some possible synonyms include evade customs, bypass security, smuggle merchandise, or illegally transport goods. Other related phrases might include sneak past authorities, circumvent security measures, or elude detection. Regardless of the specific phrasing used, the core idea is that someone is trying to get past customs or security without proper authorization or documentation. This can be dangerous and illegal, and can result in serious consequences for those involved, including fines, imprisonment, or deportation.