19 CFR § 141.101 - Time of deposit.
Estimated duties shall either be deposited with the Customs officer designated to receive the duties at the time of the filing of the entry documentation or the entry summary documentation when it serves as both the entry and entry summary, or be transmitted to Customs according to the statement processing method as described in § 24.25 of this chapter, except in the following cases:
(a) Merchandise released under entry documentation. In the case of merchandise released under the entry documentation listed in § 142.3 of this chapter before filing of the entry summary, deposit of estimated duties shall be made at the time the entry summary is filed unless the merchandise is entered for warehouse. If the merchandise is entered for warehouse, estimated duties shall be deposited in accordance with paragraph (b) of this section.
(c) Informal mail entry. In the case of merchandise entered under an informal mail entry, duties shall be paid to the postal employee at the time he delivers the merchandise to the addressee (see part 145 of this chapter).
(d) Appraisement entries. In the case of merchandise entered under an appraisement entry, deposit of estimated duties shall be made immediately after notification by the appropriate Customs officer of the amount of duties due.
(e) Entry for transportation or under bond. No deposit of estimated duties is applicable in the case of merchandise entered for transportation or temporarily imported under bond, entered for permanent exhibition under bond, entered for a trade fair under bond or entered under bond for similar reasons.