in my experience here in the U.S. if I receive an antique sword, properly described as over 100 years old, I am not charged and duty if it is sent through U.S. Mail (or comes through U.S. Mail here), if it comes UPS, FedEx, or any other shipping method I am charged a duty. In the past I call and inquire about the duty and explain the item is antique and no import tax/duty is due, I am told I can dispute the charge but the cost of disputing is more than the duty. So I usually ask sellers or others to send me items though the Post.
I also have discovered that import and export or swords with same' or horn, or any fish or wildlife materials requires a special permit. It is legal (so far) to send and receive these items but they must follow U.S. laws and regulations. It seems the U.S. Government quietly started enforcing these requirements (already on the books) in 2014. So be carful! They wanted to confiscate a gunto mounted sword I mailed out of the U.S.