That's not true. You do have to add the sticker on the outside to show there's a lithium battery inside but you can send them. They usually travel by ground instead of air.
I know they always ask you if there are lithium batteries in the package you are sending, and just to be sure before I posted my comment, I searched last night using the term: Can you mail lithium batteries through the US postal service, and the response came up NO. After seeing your response just now, I did the same search, and got the answer YES. Thanks for pointing it out.