Here is the tag required by the Feds, but I may suggest you put as much info on the tag as possible. I would definitely put down the hunter's license number on the tag, just to prove that it was legal in the first place, and there is some traceability.
§ 20.40 Gift of migratory game birds.
No person may receive, possess, or give to another, any freshly killed migratory game birds as a gift, except at the personal abodes of the donor or donee, unless such birds have a tag attached, signed by the hunter who took the birds, stating such hunter's address, the total number and species of birds and the date such birds were taken.
§ 20.44 Marking package or container.
No person shall transport by the Postal Service or a common carrier migratory game birds unless the package or container in which such birds are transported has the name and address of the shipper and the consignee and an accurate statement of the numbers of each species of birds therein contained clearly and conspicuously marked on the outside thereof.
§ 20.39 Termination of possession.
Subject to all other requirements of this part, the possession of birds taken by any hunter shall be deemed to have ceased when such birds have been delivered by him to another person as a gift; or have been delivered by him to a post office, a common carrier, or a migratory bird preservation facility and consigned for transport by the Postal Service or a common carrier to some person other than the hunter.
There is the legal stuff. I suggest U.S, Postal Service. You can't insure the package because it is a duck, but they have never failed me in the past. So what you really need to do is Freeze the bird solid, double-wrap the bird, and ship it a well insulated box. It is best if shipped in a styrofoam cooler, but if you have the two zip codes, you can call USPS, and see if it can be delivered in one day, which will alleviate a lot of the insulating concerns. remove as much air pockets in the box by stuffing with newspaper, and this will act as a great insulator. of course shipping in the heat of the summer is something I would avoid. Sometimes they will say it SHOULD ship in one day, but we guarantee 2 or three. I would definitely pay the extra shipping cost and have it delivered asap. In other words Priority Shipping. The box should be lined on the inside as well with a garbage bag to prevent any kind of water leaking from the thaw that may occur. The box has to be marked with the bird on the outside of the box, per rule above. The foot has to be tagged per rule above, and lastly, the bird becomes part of your possession as soon as it arrives to your home, and as long as it is in your freezer, (even with the flesh removed, it still remains and counts against your possession limit. If it is a bird you don't shoot in your area, I would suggest keeping the tag in your possession even after it is mounted. I'm not sure if it is the law, but again, there must be some traceability.
Any other questions, please feel free to ask.
Hope this helps,