A button should be added to the Courier, for a total of 5. (t)
This button would be called "Transfer". When you press it, if the Courier is at the fountain, all items in your bank will automatically be put on the Courier.
This is helpful for when you buy items but the Courier isn't at the fountain, so all the items get put in your bank instead. When the Courier comes back to the fountain, you hit one hotkey to Transfer the items to the Courier and you're done. It would reduce alot of clicking and time.