Acme, Inc. instructed its bank, AJM Bank, to pay $500,000 to TreeTop, Inc. TreeTop, Inc. was a customer of First State Bank. AJM Bank executed Acme, Inc.'s payment order by issuing its own payment order to First State Bank, and instructing First State Bank to credit Treetop's account with $500,000. AJM Bank could properly be described as a(n):