Requirements:
2 tossable items without 255 quantity
1 tossable item with 255 quantity
NO KEY ITEMS IN INVENTORY

Method:
Order items so that your 2 tossable items are in slots 1 and 2 and 255 item is in slot 3
Toss everything below the 255 item
Toss the top two items so you have 3 255 items
Swap items 1 and 2 and slot 2 will become 99 (x5, 99, 255)
Toss all but one of item 1 (1, 99, 255)
Swap item 2 and 1 (99, 1, 255) (YOU MUST SWAP 2 with 1 NOT 1 with 2, THIS IS THE ONLY PART YOU MIGHT SCREW UP)
Toss all but one of item 1 again (1, 1, 255)
Swap item 1 and 2 (2, 255)
Swap item 1 and 2 again 

UNDERFLOW ACHIEVED!