This personally has never happened to me, but it did to a friend of mine. Best advice I can give you is what the guy above me said, Just ignore it and take the loss versus risking legal action. When importing replica merchandise, I always assume the possibility of a seizure. If you have a decent supplier though, sometime they will reship or refund. I know mine will if this ever happens.
They probably just did a random check. They will probably flag your name, just use a different name, and a new address. I hear the East coast customs are a lot easier to pass thru than the west coast. just a tip.
Just tell them you didn't order any caps? I don't think they can prove all the other shipments you ordered, were replicas. Or tell them you thought they were authentic maybe? I'm not sure.