Don't fight. Hang up and call back and talk to another rep. Ops, that's advice for calling customer service at Citi when you have a problem.
Why not just go elsewhere? You are very unlikely to win a battle like this with a credit union--and, why bother? Demanding a written policy from a credit union employee is unlikely to result in a good result here.
But, good luck.
PS If you read this thread (put aside several weeks) you would discover that they don't have to take your coins--legal tender or not.