Under the "Bundles" menu item, you can add tracks to your release using the note icon. In the popup window „Add songs to the bundle“ you can find the „Bundle Only“ function.
„Bundle Only“ means that some of your songs or all your songs can only be purchased as a complete album, not as single tracks.
Please note that this can affect your sales negatively. Consider that iTunes
doesn’t support the „Bundle only“ function and your album won’t be listed in the shop.