It's possible that you've heard someone refer to it as a "Yorkie" informally, perhaps as a nickname or local slang. However, this is not a standard term.
The Royal Mail uses various types of cages and containers for mail, depending on the size and weight of the items being transported. These containers are typically made of metal or plastic and designed to be secure and durable.
If you're interested in learning more about the specific types of cages used by Royal Mail, you could contact them directly or research their logistics and transportation operations.