@Yogi 

I'm not sure if this is exactly what you're asking, but you can type the weight in pounds in the DESCRIPTION box on the invoice/sales receipt, and then, to have the correct amount in tons show in the QTY box, enter the weight in pounds divided by 2000.  For example, if you're selling 123,456 pounds of gravel, enter 123456/2000 in the QTY box and QBO will calculate the correct amount in tons (61.728 tons).  You can round as needed.  If the gravel is an inventory item, then you want to make sure that your inventory is in tons as well.