Personally the pop-up which asks how many items to put into the mail is kinda unnecessary at least for me.
An option in the gui controls to disable this pop-up would be nice.
Experience players know they can hold control to drag and drop the ammount they wish.
Perhaps a little tutorial for new players how to do this might be nicer. Not all players follow tutorials but that is their problem

A smart player will just ask on the zone chat so that will suffice.