Your inmate does not need any credits or an InmateAid account to write back to you. The process on their end is simple and costs nothing beyond a postage stamp.
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 InmateAid address, and mails it out through the facility's regular outgoing mail. That is all they need to do.
On InmateAid's end, when the reply arrives it gets scanned into your account and you receive a notification that incoming mail is waiting. You log in and retrieve it from your dashboard. The fee for that incoming mail service is $1.99 per letter.
The reason a lot of people use this system is exactly what you might expect. Your home address never appears anywhere in the exchange. Your inmate writes back to a Florida address, not to wherever you actually live. That privacy protection works in both directions and is one of the more practical features of the service for people who have safety or personal reasons for keeping their location private.
To set it up, make sure your account is active and your inmate has the InmateAid return address, which will be printed on every envelope you send them. Once they have that address they can write back at any time, and you will be notified in your account when something arrives.