The $9.00 was a flat fee per device for the ENTIRE cruise (not per day), whether it's 4 days or 14 days. Note that this price is only for on-board texting and phone call using the app. Actual outside internet access is much more expensive.
If you're on one of the ships that supports the app I'd recommend pre-loading the app to your deivce ... it won't do anything though till you're on-board. Once you're board put your device into airplane mode, turn on the wifi and connect to the RoyalWifi which should pop-up. Once you connect to RoyalWifi and open the IQ app you can enter your name and reservation info which will then give you free access to your account, schedule, reservations etc ... sort of like My Disney Experience for Royal. On the app there will be text phone buttons. This will give you a message about the cost of the service and allow to activate that device and charge to your shipboard account. Any device you will communicate with will also need to have the text/call feature activated but it works between iphone and android. The app can be a little glitchy, but for the price it's great way to keep in touch on the ship.
If you're buying a full on internet plan, I would just use a wifi based text app like iMessage.