Thank you for submitting your query,
Unfortunately at the moment, packing slips are designed to print all the items on a Sales Order and not just a selected few.
As you have already inferred Shipments and Picking do not display never diminishing items.
This is due to the fact that Never Diminishing Items are product codes that do not require physical inventory to be stocked. These are used for commonly incurred charges such as freight, taxes, or shipping fees.
I can definitely understand why you would like to print a single product line on a Shipment, Picking, and Packing slip.
I have forwarded this to our products team as an improvement suggestion, however, we cannot provide an estimate on if or when this feature will be implemented.
In the meantime, you can follow our product improvement releases by click here
Please do not hesitate to contact us should you have any further suggestions or questions.