5 CFR § 1201.84
Proof of service
November 4, 2020
The person who has served the subpoena must certify that he or she did so:
(a) By delivering it to the witness in person,
(b) By registered or certified mail, or
(c) By delivering the subpoena to a responsible person (named in the document certifying the delivery) at the residence or place of business (as appropriate) of the person for whom the subpoena was intended.
The document in which the party makes this certification also must include a statement that the prescribed fees have been paid or offered.