I have had great experience with metzer. He stands by his birds and his business 100%.
If you lost just one, they won't be able to ship a replacement safely by itself. What they will do is either refund your cost for the one bird, or if you plan on ordering again from them they will add it at no cost to your next order-- even if its not until next year.
Simply call them up and report your loss, and if you want the "raincheck" on the duckling rather than the refund they'll tell you to phone in your next order and remind them of your freebie when you do.
I don't know how many you ordered, but they generally do not include extras on small orders-- a lot of people order few ducklings because they only have room for the number they order.