28 CFR § 540.21 - Payment of postage.
(a) Except as provided in paragraphs (d), (e), (f), and (i) of this section, postage charges are the responsibility of the inmate. The Warden shall ensure that the inmate commissary has postage stamps available for purchase by inmates.
(c) Inmate organizations will purchase their own postage.
(d) An inmate who has neither funds nor sufficient postage and who wishes to mail legal mail (includes courts and attorneys) or Administrative Remedy forms will be provided the postage stamps for such mailing. To prevent abuses of this provision, the Warden may impose restrictions on the free legal and administrative remedy mailings.
(e) When requested by an inmate who has neither funds nor sufficient postage, and upon verification of this status by staff, the Warden shall provide the postage stamps for mailing a reasonable number of letters at government expense to enable the inmate to maintain community ties. To prevent abuses of this provision, the Warden may impose restrictions on the free mailings.
(f) Mailing at government expense is also allowed for necessary correspondence in verified emergency situations for inmates with neither funds nor sufficient postage.
(h) Mail received with postage due is not ordinarily accepted by the Bureau of Prisons.
(i) Holdovers and pre-trial commitments will be provided a reasonable number of stamps for the mailing of letters at government expense.
(j) Inmates may not be permitted to receive stamps or stamped items (e.g., envelopes embossed with stamps, postal cards with postage affixed) other than by issuance from the institution or by purchase from commissary.