COD charges should be returned if the mail is returned.
On the EU, people are 'trolling' in zone chat, offering high amounts for alchemy regents and then returning the CODs, thus ripping the (usually new) players off of the COD cost.
Also, if someone makes a mistake and CODs the wrong item (recipes that people are searching for in zone chat for instance), the 'nice' person that receives the wrong item is compelled to pay the cost of the COD after returning the wrong item.
Seriously, this is a jacked-up system and needs to be addressed.