19 C.F.R. § 132.23 – Partial release procedure.

(a) Notification of quota restrictions. If because of quota restrictions, a mail importation cannot be released, the director of the port of destination shall notify the addressee on Customs Form 3509 of the procedure required by paragraph (b) of this section, and shall inform the addressee that upon return of the Acknowledgement of Delivery by Postal Service, the packages admissible under the absolute quota will be forwarded to him and the restricted packages will be returned to the sender as inadmissible. The port director may at his discretion hold packages if it appears that the absolute quota will reopen in less than 30 days.
(b) Acknowledgement of delivery. An Acknowledgement of Delivery by Postal Service shall be sent to the addressee. He shall be advised that if he desires to secure release of less than the total number of packages of the merchandise, the Acknowledgement of Delivery by Postal Service must be signed by him and returned to the port director. Such Acknowledgment of Delivery by Postal Service shall be in the following form:
Acknowledgment of Delivery by Postal Service In consideration of the fact that certain articles in a mail importation consisting of (state number) packages mailed to me by ________________________ (name of sender) of ____________________________ (address) on ________________ (date of mailing), are subject to quota restrictions under which only a portion of such articles may be admitted to entry at one time, and the Postal Service permits no division of the importation before delivery thereof, and since I am desirous of receiving the packages of such importation which are admissible to entry under the quota administered by the United States Customs, I hereby agree and acknowledge that delivery of the package or packages to the United States Customs shall be regarded as delivery by the Postal Service to me. (Signature of addressee)
(c) Agreement to less than full delivery. If, in any case, the sender of a mail package has indicated his agreement to the delivery of less than the entire importation at one time, an Acknowledgment of Delivery by Postal Service need not be secured from the addressee.
(d) Deposit required. If a portion of a mail shipment may be released, the port director may require a deposit of an amount sufficient to defray the expenses of repacking merchandise for shipment by mail to the addressee. The shipment shall be under Government frank without new postage.