Inertia is known to be the resistance of the object in order to change what it is doing or where this is. If there would be further change or more effort will be done, then the milk carton might spill but this time, the milk carton has a mass that makes it stay on the surface even when the paper underneath the milk carton is removed.
It will also depend on how fast the paper will be removed though. If it would not be removed fast, the milk carton may fall. It will always depend on a lot of factors.