Mailing gold cross-realm isn’t an option. There are ways to transfer gold but they’re going to (mostly) cost you:
-Load up 1 mil on a character and transfer the toon to the target realm, then send the gold from the transferred character to your intended character.
-Load up a guild bank and do a guild transfer. This is much more economical since you can (someone correct me if I’m wrong) transfer 10 mil gold this way to the target realm. Once transferred, you’d then send the gold to your intended character.
-There are various account-bound items (I don’t know the exact names, sorry) that you can buy off a vendor, send cross-realm, and sell back, but you will take a heavy gold loss in the process (at least 50%+ I believe from what I’ve heard). Again, someone correct me if I’m wrong here.
-Finally, you can cage pets and send them cross-realm, then resell them on the target realm’s AH. Much slower way to “transfer gold” but it’s a tool in the kit and you may even come out with more gold in the process.