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There can be two reasons.

The paperboard box or thick paper pulp container box contains the soft drink. We have a small hole at the top and we drink beverage using a straw.

After the beverage is finished, the shape of the box is remains same. It is not compressed because

1. The air pressure inside the paper pulp box is same as that of the air pressure outside the walls. The walls are pushed by air pressure = 1 atmosphere, on both sides. So it retains shape. As we drink, we pause now and then. At this time air enters inside and fills the empty space created by our drinking.

2. The stiffness of the paperboard/pulp material resists the deformation.

If you suck air through the straw very hard, then the box gives in. It gets compressed. This is due to evacuation of air inside. The air pressure inside the walls becomes less compared to air pressure outside the walls.

If you blow air into the box, it retains original shape and even it bulges out. It is due to excess pressure on the walls inside the box.

In simple words, the carton or the tetra pack is stiff enough to stay intact even though their is nothing inside it.