They'd have a hard time getting them from U.S.-based customers: souls, being human remains, cannot be sent by US Mail.
(http://improbable.com/airchives/paperair/volume6/v6i4/postal...)
(http://improbable.com/airchives/paperair/volume6/v6i4/postal...)
I don't think the rule specified that it had to be physical remains, although that would certainly make it easier to package.
And if not physical remains, why would anyone be worried about whether the post will ship it?
Like this, perhaps? http://news.ycombinator.com/item?id=1270582