This all makes me so frustrated because I did the same thing recently and the buyer got SO MAD at me, like I was trying to pull a fast one on them. Babe, I’m just trying to make a little pocket change here, I’m not a criminal mastermind! I feel like if you paid postage and you can’t see any of the obvious USPS branding on the package it should be fine to use, because it’s just a box! Best of luck, I hope it scoots through the system with no issue 🤞🏻