19 CFR 143.28: Deposit of duties and release of merchandise.

Unless statement processing and ACH are used pursuant to § 24.25 of this chapter, the estimated duties and taxes, if any, shall be deposited at the time the entry is presented and accepted by a Customs Officer, whether at the customhouse or elsewhere. If upon examination of the merchandise further duties or taxes are found due, they shall be deposited before release of the merchandise by Customs. When the entry is presented elsewhere than where the merchandise is to be examined, the permit copy shall be delivered through proper channels to the Customs officer who will examine the merchandise.