Stay on the right list
An Amazon carton that is slightly large still has to be an Amazon carton if that is the shipment. Fill the gap with loose fill, paper void fill or Ecoflo peanuts, which are corn starch and described as biodegradable.
Switching to a removal cube because it needs less fill sends the wrong carton. The families are not interchangeable.
What the eco page adds
Recycled air pillows and recyclable paper void fill are on the eco-friendly page. Bubble wrap is the surface cushion, not this gap.
A closer size in the same family is still the better first step.