Items cannot be dragged to the bottom of the Quick Buy queue; they can only be dragged down to the 2nd-to-last item. The thin white line denoting where it will be placed does not appear at the very bottom.
Fortitude cannot be dragged to the bottom:

Fortitude can be dragged to the 2nd to last slot:

However, the last item in the queue can be added to the top of the queue without issue.
Slowing Bullets can be dragged to the topmost slot:

Fortitude cannot be dragged to the bottom:

Fortitude can be dragged to the 2nd to last slot:

However, the last item in the queue can be added to the top of the queue without issue.
Slowing Bullets can be dragged to the topmost slot:
