Why not just go with a square carton design? Now the whole rectangle is full and they're mostly recyclable (I think?). It would probably also normalize Soylent into something like milk.
(Paper) cartons are much weaker and less resilient than plastic bottles. Think about the number of times you've seen dented or folded chicken soup cartons on supermarket shelves... I've certainly seen plenty of Safeway soup boxes to avoid because of obvious exterior damage.
I don't think cartons would get them very far in that area.
A few cartons being damaged in the school delivery is fine, but even one damaged in the Soylent delivery means an angry customer.
"So why sqround? Why not a crisp cornered square bottle, you ask? Obviously, a perfect square would be most efficient in terms of packing. Squares, however, come with their own set of challenges. When bottles travel down the manufacturing line on their way to your door, they are constantly moving from single file to big groups, around curves and corners, and jostling into place. A bunch of round bottles will roll off one another and travel smoothly around corners, but a bunch of square bottles have a tendency to lock up with one another, kind of like a losing game of Tetris. By generously rounding the corners of a square (to the industry-coined “sqround”) we solve these production line issues."
It's a bit like people go and pay $5 for a cup of coffee in a coffee shop. They are buying the experience more than the coffee.
Slightly OT but for me it's the reverse. If I go to a really good gourmet café then yes it may be expensive and yes, a component of the price is the experience (usually these cafés are nice places to hang out in) but most of it is the coffee itself. I cannot buy a coffee that good for less money in another place, hence I am willing to pay the price they are charging.
I would also think filling and distribution of a carton is a solved problem considering how much milk/juice gets moved in them without too much damage.