There is no automated delivery confirmation on individual letters, and that is true of the postal system generally rather than anything specific to InmateAid. Once a letter enters the facility's mailroom, the staff processing and distributing mail does not send delivery receipts back to senders.
The most reliable signal that your letter arrived is hearing from your inmate directly. When they receive your letter they will typically mention it on the next call or write back. That response is your confirmation.
On the rare occasion that a letter gets returned, InmateAid handles it. If something comes back, the team investigates why it was rejected, fixes whatever the issue was, whether that is an address problem, a formatting issue, or something else, and resends it at no additional charge. That backstop is part of what makes the service reliable.
InmateAid has been doing this since April 2012 and has processed a significant volume of mail over that time. Returned letters are the exception rather than the rule, and when they do happen the resolution process is straightforward.
If you have not heard from your inmate after a reasonable amount of time and are concerned the letter did not arrive, reach out to InmateAid's support team. They can look into the status on their end and let you know if anything came back or if there is an issue worth addressing.