Just like in some previous games from this series, Kingdom Hearts 3 has postcards which can be traded for valuable rewards (e.g. Orichalcum). Check this page to learn how to get postcards and where you can send them.
How to get postcards in Kingdom Hearts3?
You get postcards by making purchases in Moogle Shops scattered in various worlds. Since you get them by making purchases, some of them are obtained randomly. Just keep buying items - you are guaranteed to get a few of these items.
How to send Postcards in Kingdom Hearts 3?
To send a postcard, go to Twilight Town (Tram Commons area). A red mailbox stands near the Moogle Shop. Interact with this object - Sora will send postcards automatically. Sending postcards gives you valuable rewards, e.g. Orichalcum. Sadly, the rewards are random so it is possible that you will have to send a lot of postcards to get the most valuable rewards.