Two good questions in one, so let's take them separately.
On the reply process, your inmate does not use the same service to write back and it costs them nothing to respond. When you send a letter through InmateAid, the return address on the envelope is InmateAid's Florida address. Your inmate writes their reply on paper, addresses it to that Florida address, and mails it out through the facility's regular outgoing mail with a postage stamp. InmateAid receives it, scans it into your account, and sends you a notification that incoming mail is waiting. You retrieve it as a PDF from your dashboard. The fee on your end for that incoming letter service is $1.49. The inmate pays nothing.
On the transfer from Franklin Parish to East Carroll Phase 3, early release is not directly what this signals, but it is potentially good news in terms of trajectory. Transfers between facilities in Louisiana's system often reflect a step-down in custody level, meaning your person has earned enough trust through good behavior and program participation to be moved to a less restrictive environment. East Carroll Parish Correctional Center operates phases of programming and custody levels, and moving into Phase 3 suggests progression through that system.
That kind of movement is the path toward the earliest possible release date, even if it does not mean immediate release. Each step down in custody level builds the institutional record that supports eventual parole consideration or release at the earliest eligible point. Encourage him to keep that momentum going by staying clean and engaged with whatever programming Phase 3 requires.