I’d just hit up Amazon or your local Staples and look for a box of rigid mailers like this. https://www.amazon.com/9-75x12-25-Cardboard-Mailers-Shipping-Envelopes/dp/B07L9CGR6V/ref=mp_s_a_1_10?keywords=comic+book+mailers&qid=1565118728&s=gateway&sr=8-10

In the long run it might not save a ton over priority mail but a couple bucks a book is still a big deal. Looks like under “normal” mail rules you’d pay $4-$5 to ship each one out as a parcel. It’s classified as a parcel if it’s rigid and non bendable. So with supplies included you’d still be ~$2 under the Priority price.

I’d definitely request media mail. From what I’ve read it can really just vary depending on the post office and clerk unfortunately, but it could really help cut the cost.