Welcome aboard, Kathleen! You're officially a Castaway Club member! Isn't that wonderful? Cruising with Disney for the first time must have been great for your family, but you'll find some extra perks now that you're cruising as a Castaway Club member.
One of those perks includes the option to begin booking extras up to 90 days prior to sailing, as you mentioned. However, your cruise must be paid in full to begin booking these extras. Though it is not necessary to have the cruise paid off until 75 days prior to sailing, I recommend trying to pay it off by 90 days prior so you're able to take advantage of that early booking of extras. These extras like
Port Adventures,
Adult Exclusive Dining and Spa Treatments do fill up sometimes, so the earlier you are able to book those, the better.
I hope that clarifies for you the cruise payment and booking of extras, Kathleen. Stop back again if you have other questions before you set sail! I'm happy to help!