It looks like the data types for those columns are set to varchar instead of int or double. That would make it difficult to do number type of comparisons.
I think >= is the one that makes the most sense. I would think you would be saving the maximum weight and maximum volume for a given shipping option. That would mean you would want the value in the cart to be less than that maximum for the shipping to be applicable. Then you would sort by the cost ASC because you would want the lowest cost that was applicable for that size shipment.