If you order items via Service from Blacksmith, and your inventory is full, you can't drag the item into your inventory, BUT you can "put it back" into the collection box. If you exit out of that menu, the items that didn't fit in your inventory will be on the ground. I deleted some grass from my inventory, and was able to pick up the item on the ground AND from the collection box so you can get 20 iron screws for the price of 10.

This bug appears to be endlessly exploitable. Have not tried with other vendors.