Best way to pack a comic book for shipping CGC grading

While I was auctioning off my comic books on eBay, I came to a dilemma on how to ship a comic book without damaging it. My 2 options for shipping are USPS 1st class for cheap price comics or USPS priority mail. So I turned a USPS priority flat rate box into a custom crush or bend proof box for shipping out the comics. I have since sold 25 different auctions without a problem, just a lot of praise about the packaging on my feedback.