I don't know your country to give specific info, but if it is something like in mine, this could be issued by your local National/State Food Authority. For example in Greece it's EFET (similar to FDA) or the State Veterinary Agency. An exporter has to fill some documents in one of the two, whoever is closer, stating his credentials, country of export, what and how much etc, an official verifies all statements and issues something like a "certificate" which accompanies the shipment.
It could be something similar. A private company/CB could propably issue this too, but I don't really think it would be readily acceptable, unless the importer is also ok with that (or doesn't really care as long as he has something to show!!!).
There is also a slim chance he could mean the weight and volume that is measured from the shipping company upon reception of the goods and could be written on the invoice and want a copy of that ???