You know to group things together for giving to others - or even MB sales.

Maybe in sizes, 2, 3, 4, 5 and multiples of the 2, 3, 4, 5 - where they take up the same about of space in your inventory as their size - all of which are rectangular in shape.

Players should by default of course be able to see into the boxes /w the option to hide the items from view for surprise gifts.