A lot of people do not understand what COD mails are or how they work. COD stands for:
Cash On Delivery
In game terms this means you can mail items to somebody with a credit amount attached and they can't take the items from the mail without agreeing that the system can automatically send you the credits from their inventory.
How To Send A COD Mail
Tick the COD option in the bottom right of the mail window
Enter a credit amount
Attach the items you want to sell
Fill in the To, Subject and Message fields
When they receive the mail and try to take the items out of it they get a confirmation box. If they click to agree then the credits are automatically mailed to you.
What happens if they delete the mail or decline it? The mail and items are returned to you.
What happens if they ignore the mail? It should get returned to you in 7 days (might be 14).
I sent the mail and they haven't received it yet?! It can take 30 minutes for mail to arrive. That includes your return mail with the credits attached.
What happens if I forget to tick the COD option? Then you end up mailing the other person the items and the amount of credits you meant to request! If that happens then you have to hope they will send you the credits back along with the amount you meant to request.
Check your credit amount! You can't change the amount once the mail has been sent. In the first screenshot above you can see something I do whenever I deal with credits which is dead simple. Just use the mouse to select three zeros. It makes seeing the actual amount much easier when you are dealing with millions or tens of millions.