First up, create your Package and add the items that you will always include under Contents, and any optional items as Accessories.
Contents will be automatically added when you add the package, but Accessories is the perfect place to put any optional or add-on items you may include with your package.
For example, a Casino Table will always require chips, the table, and the cards, but you may offer custom branding or extra chips. The branding and extra chips will be added as Accessories.
Now when you go to add the Package to a project any Contents will be automatically added, and you will have the option of adding the Accessories by clicking next to the Package's name.