FlexJobs are not an add-on feature. You do not need to pay extra to use FlexJobs.
They are an integral part of any OnCall Air setup, so there is no way to hide the FlexJob tab from a proposal builder.
The tab will show up even if you have 0 published FlexJobs.
However, your team will not be able to search or select any FlexJobs until you publish at least one in the Office catalog.
Check out how to get started with a single Placeholder FlexJob!