You MUST affix a stamp to any letter being sent into a federal prison or any facility. None of them accept US Mail and pay the postage, you have to.
https://www.inmateaid.com/ask-the-inmate/to-mail-out-a-letter-to-my-mom-at-the-coleman-satellite-camp-in-florida-do-i-mail-it-with-a-stamp-or-just-the-coleman-camp-address-with-no-stamp-nbsp#answer
Accepted Answer Date Created: March 06,2019