No notification system is needed on your end. The letter handles it.
When you send a letter through InmateAid it goes out as physical mail through USPS, printed on real paper and mailed to the facility. It arrives at the mailroom, goes through the standard inspection process, and gets handed directly to your inmate at mail call. That moment is the notification. There is no electronic alert, no staff announcement, no system message telling them something is coming. They simply walk up to mail call and a letter is there with their name on it.
This is meaningfully different from digital messaging systems like HomeWav or JPay, where an inmate might need to log in to check for new messages or where a notification appears on a tablet or kiosk. InmateAid's letter service is entirely physical. Your inmate holds the letter in their hands, sees your name on the return address, and that is all the notification that happens.
You do not need to warn them in advance or tell them to expect something. If you want to, there is nothing stopping you from mentioning on a call that you sent a letter, but it is not necessary for the mail to be received and read.
The one practical note worth keeping in mind is that mail call timing varies by facility and housing unit. Letters typically arrive at the mailroom two to three days after InmateAid mails them and are distributed at the next mail call after inspection. Depending on how often mail call happens at your person's facility, there may be a day or two of additional wait between arrival at the mailroom and the letter reaching their hands.