Agent not authorized to collect

What does Agent not authorized to collect mean in American Law?

The definition of Agent not authorized to collect in the law of the United States, as defined by the lexicographer Arthur Leff in his legal dictionary is:

A term which, at least if so printed on an invoice as reasonably to constitute notice to a buyer, is sufficient to warn him not to pay the agent, and to render him liable to pay again to the seller should his incautious payment to the seller’s agent not reach the seller.