[Enhancement] Allow users to exclude a product from a package or a label from being purchased

[maxlaf]

There's no way to only ship certain items in an order at one time. If an order has 2 items and the merchant wants to ship 1 now and 1 later, it isn't possible unless they manually edit the order, remove the item they don't want to ship, create a label, and then add the item again.

I think there are 2 ways to approach this:

1. We could allow users to exclude an item from a package:
   

2. We could allow users to unselect a package from label purchases. They could put items to be shipped in 1 package and items which aren't ready to be shipped in another package, and only purchase 1 label: