1) counter sample - что имеется в виду: арбитражная проба, контрольная проба?

В документе излагаются требования для поставщиков сырья (X – производитель, которому сырье поставляется)

According to specific agreements Supplier shall delivery to X samples for analysis before shipment in order to perform acceptance of the Material. X shall have the right to perform analysis on material at factory gate in order to check correspondence between the Material and samples delivered.
A COUNTER SAMPLE of material claimed for analytical reason will be kept by X for six months from date of claim available for supplier for analysis.
In case of different results between X and Supplier lab a third lab certified ISO 17025 will be involved to solve any doubt. Cost of this third lab will be on charge of the part with negative results.

2) Total gross commercial weight - это просто общий вес брутто или здесь что-то еще подразумевается?

The supplier will have to produce the shipping notice for every supply lot, giving the following indication
Type of material (trade name)
ASTM code if present
Lot number
N° of packages
Net weight of each package
Total net weight
Total gross commercial weight
References concerning the means of transport (transport company, type of truck and number plate)

Посмотрите здесь:
http://forum.wordreference.com/showthread.php?t=562348&langid=18
и здесь:
http://mymemory.translated.net/t/Italian/English/controcampione