There is no real-time delivery confirmation, and that is true of the US Postal Service generally rather than anything specific to InmateAid. Once a letter enters the mail stream and passes through a facility's mailroom, there is no automated receipt sent back to the sender. That is simply how physical mail works.
What you do have is a reliable system with a track record going back to April 2012. InmateAid has processed an enormous volume of mail over that time with a high level of consistency. Your account dashboard keeps a complete record of everything you have sent, every letter, photo, magazine order, and phone interaction, so you always have a reference point for what went out and when.
If a letter comes back for any reason, it comes back to InmateAid's return address in Florida. When that happens, the team investigates why it was rejected, resolves the issue, and resends it at no charge. That backstop means a returned letter does not simply disappear.
The most practical confirmation you will get 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 real world delivery confirmation and it usually comes within a week to ten days of sending.
If you have not heard anything after a reasonable amount of time and are concerned, reach out to InmateAid's support team. They can check the status on their end and let you know if anything came back or requires attention.