One simple solution is to ask to be weighed after dropping off the non-ferrous as well. This figure gives the total weight of all materials and would be easy to highlight their mistake. I always get a total weight because the platform scale could be different than the truck scale. This provides bargaining material and hard figures to negotiate.
I know scales are supposed to certified in most states at least once a year. Did you know there is a margin of error that is acceptable to the DOT. A platform scale for example could be zeroed in at 100 lbs. but a lb. off at 175 lbs. An old trick was to jam pennies under the platform after certification. Prior to the next certification the pennies would be removed ensuring the scale weighed light during the interim. If there are a bunch of pennies on the ground by the non-ferrous scale, I would question the honesty of the business. Without a total load weight it is easy to manipulate figures to make them fit.
As far as their attitude I think dropping a hint about calling the DOT and the BBB might change their perspective as to your value as a customer especially if the weight totals do not match.
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