How to collect money due from major company, I'm bad at it?

A major book publisher handling a software a project I helped develop, requested some software modernization, in a somewhat informal way. I was told after delivering and invoicing that the department did not have the funds, signed and returned a retroactive contract to get things going more formally last year, never have heard back. I want to reach beyond the department that commissioned me, sending copy of the contract and complaint to accounts payable and CEO to get their attention.