Make your final payment with your TA as you usually do. I am in my 70’s and found making purchases such as drink pkgs, etc are easier for me to do on the website. I just log in there and go to the planner.
You will have your Set Sail Pass on your app which is good but being old school I still print mine out as it speeds up checkin rather than having to look for it on your phone.
The only thing I use the app for is checkin, muster drill, health questions, to keep up with onboard daily activities and to keep a check on my account onboard.