Depending upon the facility, there is a provision for "indigent mail" - if the inmate has a commissary balance for a period of time (less than a few dollars) the prison/jail will offer the inmate a set number of "free stamps" along with envelopes and paper to mail letters.
https://www.inmateaid.com/ask-the-inmate/without-having-money-on-your-books-do-they-provide-stamps-or-envelopes-to-the-inmate#answer
Accepted Answer Date Created: February 03,2014