For starters, it would help to be certain that your actions are legal. I'm not certain how or if UPS rules apply to black powder replicas, but I DO know that the same Federal law that says handguns cannot be shipped USPS and must be shipped by common carrier also stipulates explicitly that the carrier must be informed that the package contains a firearm.
They are legal, and this is old news.
Black powder replicas are NOT firearms as defined by federal law, and are not restricted from shipping by anyone, including the USPS. In fact, among black powder enthusiasts, shipping by flat rate box from the post office is the preferred method. No notification is required, because although it may LOOK like a firearm, it is not. There is thread after thread on this in the cowboy action and black powder forum.