19 C.F.R. § 123.21 – Merchandise in transit.

(a) Status. Merchandise may be transported from one port to another in the United States through Canada or Mexico in accordance with the regulations in this subpart or subparts E for trucks transiting Canada, F for commercial traveler's samples, or G for baggage. Merchandise so transported is not subject to treatment as an importation when returned to the United States, and no inward foreign manifest is required for merchandise returned under an in-transit manifest. In-transit merchandise returned to the United States shall be treated as an importation as are shipments made from Canada or Mexico if:
1. (1) An in-transit manifest is not furnished for the merchandise upon its return to the United States;
2. (2) The merchandise has been trans-shipped in foreign territory without Customs supervision when the transshipment required the breaking of Customs seals; or
3. (3) The Customs inspector finds any of the Customs seals applied to the conveyance or compartment unlocked or missing.
(b) Use of certain vessels prohibited. Merchandise shall not be transported from port to port in the United States through Canada or Mexico by vessel in violation of the provisions of section 27, Merchant Marine Act of 1920, as amended (46 U.S.C. 883), or section 588, Tariff Act of 1930, as amended (19 U.S.C. 1588). (See § 4.80 of this chapter.)
(c) Regulations applicable. The provisions of this subpart shall govern all merchandise transported from one port to another in the United States through Canada or Mexico under in-transit procedures, except as otherwise provided in this subpart or in subpart E for truck shipments transiting Canada, subpart F for commercial traveler's samples transiting Canada, and subpart G for baggage transiting Canada or Mexico.