The "trade usage" of a term is a use that is sufficiently regular in a particular type of contractual setting that a party to such a contract is justified in expecting that the other party is using the term in the established way.
The use of "per copy" in a price quote in the book printing and binding business to mean a single volume is a use "that is sufficiently regular in a particular type of contractual setting that a party to such a contract is justified in expecting that the other party is using the term in the established way."