A studio may opt to pay annually for the ability to post unlimited shows. That's really all there is to it.
How to upgrade
From the studio page, studio master admins can go to the Owner Tools and follow the prompts to subscribe to annual billing. If annual billing is already active, they will see the expiration date instead.
Only the studio's creator ("master admin") may sign up for annual billing on behalf of the studio.
Multiple theatres already using Cast98 requested a way to pay annually instead of per show, citing that it is easier for them to get approval this way.