The letters you purchase through InmateAid are for sending mail to your inmate, not the other way around. Your inmate responds the old-fashioned way, through regular postal mail with a stamp.
The current postage rate runs 73 cents for a standard letter. If your inmate does not have money on their books to buy stamps through commissary, the facility is required to provide basic writing materials, including paper, a pen, and postage for indigent inmates. Nobody gets completely cut off from the ability to send mail out because of a zero balance.
When your inmate writes back, they will be sending their letter to the return address on the envelope you sent, which is InmateAid's address in Florida. Make sure your inmate has your correct contact information inside the letter itself so their reply finds you. If you want them to write directly to your home address, include it in your letter. If you prefer to keep your address private, let them know to use the InmateAid address and that you will retrieve it from there.
The nine letters you purchased are yours to send at your own pace. Use them knowing that each one you send gives your inmate a reason and a way to write back.