“However we are not being charged by the ton or any other form of weight. We are being charged by the unit. ”
Right, and a unit of an RM (1m3) is exactly the same regardless of the RM type. If you want me to move a box, my profit margin on a 1-meter-cubed box is going to likely be exactly the same regardless of what is in that box. It takes exactly the same amount of space on my truck to move whatever is inside that box, which is really all that matters (the space on my truck).
If you want to make a suggestion I do invite you to come up with an actual compelling reason for the change, because its current simplicity is somewhat of a virtue.