I saw this in https://www.shapeways.com/materials/strong-and-flexible-plas tic "... Details on pricing / Density discount We also offer a volume and density discount on Strong & Flexible materials. For models that have greater than 10% density (material volume divided by bounding box volume), volume above the first 20cm3 is calculated with a 50% discount..." I made the walls of a model thicker in order to increase the density above 10% but didn´t notice a discount. When does a model qualify for this?
As it says in the quote you post, it qualifies when the material is Strong and Flexible, the volume is more than 20cm3 and the density is more than 10%.
I´ve got it. As mkroeker has answer in this post: "Not able to reduce the cost" there is a small window within we can save before the added weight outweighs the money saved through that discount. I am tweaking my model in pursuit of some balanced solution.