Yes, you are permitted to send finished, ready to use souvenirs made from cow horns to friends in the U.S. Your shipping documents will need to include a description of material. I suggest you include a statement that they are fully finished and ready to use.
So I imagine, with a little communication, people wanting to buy horn products for bows for instance, could get them shipped.
Also, there will be no problems (not that I think there has been any) should anyone purchase any of my drinking horns or beaker (and hopefully lantern panes when I get them sorted).
That said I currently have some auctions running...
http://shop.ebay.co.uk/warriorsgateway09/m.html?_nkw=&_armrs=1&_from=&_ipg=25