If you'd like to always add specific items with a package, you can add those items as 'Contents'.
What if you want your team to add additional items to that package on a project? If you wanted to have your team select items from a specific location, choose the color of the contents, for example.
You can click the three dots next to the package 'wrapper' and click to 'Add Inventory Item - Package' (you can also add 'Custom Items' to your packages).
Those items will be added as new 'Contents' of your package.