I am not sure if this has come up already but it is something I observed over a long time now. At the moment I use the latest AW2K beta but this also happened in the regular AW2K.
The problem is that at some point the link between my sold inventory and the related invoice is completely gone. I am not sure when it happens but I suspect it is when I mark an invoice shipped.
An example might illustrate it better:
I have one or more of a specific inventory and sell one (ebay or non-ebay). I create an invoice and assign the inventory to that invoice. So far so good and the invoice is nicely shown with the inventory item. But after having going through the whole cycle of receiving payment, shipping and closing the invoice upon receival that link gets lost somehow. It is then impossible to see within the inventory item to which invoice it is related. I can still go to the specific inventory item from within the invoice.

Regards,
Howard
Wheel of Time

http://www.wheeloftime.nl