7 CFR § 352.11 - Mail.
(1) Plants, plant products, plant pests, noxious weeds, and soil which arrive in the United States in closed dispatches by international mail or international parcel post and which are in transit through the United States to another country shall be allowed to move through the United States without further permit than the authorization contained in this section. Notice of arrival shall not be required as other documentation meets the requirement for this notice.
(2)Inspectors ordinarily will not inspect transit mail or parcel post, whether transmitted in open mail or in closed dispatches. They may do so if it comes to their attention that any such mail or parcel post contains prohibited or restricted products or articles which require safeguard action. Inspection and disposal in such cases will be made in accordance with this part and part 330 of this chapter, and in conformity with regulations and procedures of the Post Office Department for handling transit mail and parcel post.