I’ve taken cash in the mail when selling yoyos on the BST board. But, as the seller, I requested that the person send the cash first. After I receive the cash, I send the yoyo. With PayPal, the money is transferred immediately, so either way, the seller has the money before sending the yoyo.
I’ve also had someone take cash and put it on a gift VISA card and then use it to send money via Western Union. That worked too. A little safer than cash in the mail.
All that said, I wouldn’t purposely go behind your dad’s back to buy more yoyos. You’ll have plenty of years worth of bad decisions to hide from him in your lifetime. Don’t start just yet. Especially if you have yoyos already. My 2 cents.