Congratulations on your upcoming cruise.
The soda sticker is a set cost for the cruise.
It can run from $22-27 based on the length of your cruise.
The day you embark you will see loads of crew/bar staff selling the stickers. If you plan on getting it do get it straight away.
They will place a small sticker on your cruise card & when you go to the bar to get your soda show them the card. It is good for unlimited fountain sodas.
They do also try to sell you a coke tumbler/thermas type cup - it is small.
I usually do not get this.
I also ask for the extra large/jumbo plastic glass.
Have a great cruise.