Partial delivery?

Whenever I order some parts from you, and some items are not in stock, i get this question if i want immediate or partial delivery.

" Immediate delivery
Immediate shipment of all articles that are on hand in our warehouse, no after delivery.
  Partial delivery
Immediate shipment of all articles that are on hand in our warehouse, rest items will be shipped automatically in a separate shipment when we receive them from manufacturer.
"

But i never get anything delivered by partial delivery if I choose that, it says on the invoice i get that it is not possible!

Why?



Thomas
Norway