You do not need to have your parents permission to send mail. You do not need to be a certain age to send or buy things through mail. Walk down to the post office and put it in the blue mailbox outside the office and you are done.
You don't need a credit card. You don't need to get a money order (although you can buy one if you want). You can send checks. You can pay for Blockland in several different currencies, not just dollars. Mailing a standard letter overseas cost as much as mailing it down the street so you don't need to put $2 worth of stamps on your envelope.
There is absolutely no reason you can't buy Blockland, unless your parents don't let you out of the house, you can't get $20, or mail hasn't been invented where you live.
