19 CFR §145.71

1. (a)Relief from duties. Merchandise imported into the United States, unless nonmailable, may be exported by any class of mail without the payment of duties, if:
   1. (1)The merchandise has remained continuously in the custody of the Government (Customs or postal authorities); and
   2. (2)The mail articles containing such merchandise are inspected and mailed under Customs supervision.
2. (b)Waiver of right to withdraw. Waiver of the right to withdraw the mail article from the mails shall be endorsed on each mail article to be so exported and signed by the exporter.
3. (c)Export entry or withdrawal required. An export entry in accordance with § 18.25 of this chapter or a warehouse withdrawal for exportation in accordance with § 144.37 of this chapter, whichever is appropriate, shall be filed for merchandise being exported under this section, except for merchandise imported by mail which is either:
   1. (1)Unclaimed or refused and being returned by the Postal Service to the country of origin as undeliverable mail; or
   2. (2)For which a formal entry has not been filed and which is being remailed from continuous Customs or postal custody to Canada.