I do a lot of mailing, sometimes with stuffed bags. When I want to mail somebody some items, I drag them over to the mail-slots. However, the “fade” that the game applies to the items that are pending is so slight that many of the times, it is difficult to distinguish from normal items.
My suggestion is, make them 50% darker than their usual images, and that will make it obvious which items have been dragged, and which have not.
Thanks!