I see the problem with ""pack,"" especially if they call them that but each customer can have a different standard. Again, I have a client that buys by the ""case,"" stocks by the ""each,"" and can sell by the ""case,"" ""pack,"" and/or ""each.""
For these non-standard ""packs"" that your client has, I would have them create names like PK12, PK24, etc. It's kind of Rube Goldberg but it would work.
Again, as long as there is consistency with the weights and measures, I don't see a problem with the purchasing model you presented. If the length and weight of the copper bars they buy are NOT standard, then, yes, you have a problem.
That client with the steel bar in my example above bought several standard lengths and diameters of steel for their inventory. Each different diameter and length was it's own item code. They would use the lengths either whole or in part. The discipline I had to instill in them was what to do with ""partial lengths.