Can't COD more than 10,000g?

What's the deal with this? I'm trying to sell my Darkmoon Cards and was contacted by somebody telling me their friend would log in later today to buy one. I realized that I was not going to be on later, so I decided to COD it instead. When trying to do so, I found out that you cannot surpass 10,000g in a COD mail.

What is the reasoning behind this? This seems like an absurd limitation of the mail system that should not exist. People use seldom use COD as it is.