Those plastic balls for a ball pit. You order them in bags of 1000 pcs for 15 bucks on Aliexpress, they are bloody huge. A couple of those will do.
Especially if your goal is to mess with the delivery driver, you can max out available storage in the truck in no time.
What the hell? I ordered them to Germany and shipping cost next to nothing. Or exactly nothing, if I remember correctly. 10 bags were like $180, shipping included. Or is that some Trump era garbage? My order was about 4 years ago.