Catch weight item - # UOM

Hi all,

There is a need to have a sales unit of measure in box ( as we have trade agreements with our customers in that UOM). However we need to invoice according to weight shipped.

So my question : how do we setup the item so we are able to sell in boxes; have our agreements in lbs and still handle invoicing according to the weight shipped.

so can we have sales UOM # than invoing UOM ?