39 C.F.R. § 913.2 – Service.

(a) Service within the United States. A subpoena issued under this section may be served by a person designated under section 3061 of title 18 at any place within the territorial jurisdiction of any court of the United States.
(b) Foreign service. Any such subpoena may be served upon any person who is not to be found within the territorial jurisdiction of any court of the United States, in such manner as the Federal Rules of Civil Procedure describe for service in a foreign country. To the extent that the courts of the United States may assert jurisdiction over such person consistent with due process, the United States District Court for the District of Columbia shall have the same jurisdiction to take any action respecting compliance with this section by such person that such court would have if such person were personally within the jurisdiction of such court.
(c) Service on business persons. Service of any such subpoena may be made upon a partnership, corporation, association, or other legal entity by—
1. (1) Delivering a duly executed copy thereof to any partner, executive officer, managing agent, or general agent thereof, or to any agent thereof authorized by appointment or by law to receive service of process on behalf of such partnership, corporation, association, or entity;
2. (2) Delivering a duly executed copy thereof to the principal office or place of business of the partnership, corporation, association, or entity; or
3. (3) Depositing such copy in the United States mails, by registered or certified mail, return receipt requested, duly addressed to such partnership, corporation, association, or entity at its principal office or place of business.
(d) Service on natural persons. Service of any subpoena may be made upon any natural person by—
1. (1) Delivering a duly executed copy to the person to be served; or
2. (2) Depositing such copy in the United States mails, by registered or certified mail, return receipt requested, duly addressed to such person at his residence or principal office or place of business.
(e) Verified return. A verified return by the individual serving any such subpoena setting forth the manner of such service shall be proof of such service. In the case of service by registered or certified mail, such return shall be accompanied by the return post office receipt of delivery of such subpoena.