19 CFR 145.31: Importations not over $800 in value.

(a) Except as provided in paragraph (b) of this section, the port director will pass free of duty and tax, without preparing an entry as provided for in § 145.12, packages containing merchandise having an aggregate fair retail value in the country of shipment of not over $800, subject to the requirements set forth in §§ 10.151 and 10.153 of this chapter.

(b) The exemption provided in paragraph (a) of this section is suspended for merchandise arriving through the international postal network until such time as CBP determines that the application of the exemption is no longer inconsistent with the purpose of 19 U.S.C. 1321(a), no longer jeopardizes the revenue, and no longer facilitates unlawful importations. Notice of a modification or revocation of this suspension will be announced by the Commissioner in the Federal Register.