InmateAid costs your inmate nothing. The service is entirely free on their end and always has been.
The only thing your inmate needs to write back is a postage stamp and something to write on, both of which are available through the facility's commissary or provided to indigent inmates by the institution. They write their reply on paper, address it to InmateAid's Florida address printed on the envelope you sent them, and mail it out through the facility's regular outgoing mail. That is the extent of what it costs them.
On your end, the $1.49 fee covers InmateAid receiving the letter, scanning it into your account, and notifying you that incoming mail is waiting. You log in and retrieve it as a PDF from your dashboard. Your home address stays completely private throughout the exchange since your inmate is writing to InmateAid's address rather than yours.
The full round trip works like this. You pay to send a letter, your inmate pays for a stamp to reply, InmateAid receives and scans the reply for $1.49 on your end. That is the complete cost structure and there are no hidden fees beyond those amounts.
If your inmate has no money on their books for a stamp, most facilities provide indigent inmates with basic writing materials and postage for outgoing correspondence. Nobody gets completely cut off from the ability to send mail due to a zero balance. So even in that situation, a reply is still possible without any cost to them.