Actualy the mail will accept yeast samples, heck at work we recieve hiv, flu, salmonella, and some other nice nasty bacteria and viruses though the mail. The key though is you have to ship it properly, meaning your price goes up and theres a lot more rules and regulations. To ship it you would have to class it as a UN item, id guess UN3373 which is biological substance catagory B. By using this i dont see why you cant ship "yeast samples" by mail, and in practice it is the proper way to ship. The issue is if you are shipping it this way they can require you to disclose what your sending which you could then probably use the name of your yeast/bugs but make sure you go to the leagle weight limit on the amount you declare so your volume isnt questioned.

If my memory is actualy correct i think we got a fungus though us mail a few weeks ago.

But Fedex and UPS is much less hassel but still could give you just as much greif.