Internet buyer stopped payment on cashier's check and won't return merchandise

I have a website where I sell fiberglass fish replicas. Mr. Simmons ordered 4 replicas and insisted I send them cod. We agreed on cashier's check and I packed and shipped them accordingly. He accepted the merchandise and signed and paid. I cashed the check and then was notified by my bank that the funds were taken from my account and fines debited. He stopped payment on the CASHIER'S CHECK so my bank took the funds from me to cover the check. Now he says they are not what he wanted. He refuses to return the merchandise. He says that when I make the more expensive FULL MOUNTS that he thought he was buying, and send them to him for the same price as the replicas, then he will return my original merchandise. I offered to give him a 25% discount off the FULL MOUNTS but that he must first return the original shippment. He refuses to cooperate. I want to go after him for civil damages and legal charges. Help?