I have lengths of material as products in Ubercart, 3 and I need to be able to calculate shipping of them based on their length x width. Plenty of options for shipping based on weight but that's not what is needed here. I've got width and lengths fields in my product type already plus I have created one called "m2" to enter the square meters manually in case that is easier. Just a bit stuck as to what I should do next. Could rules help me? Maybe modifying the flat rate module or would a custom module be needed?