Maybe you should consider selling at least some unfolded. 2 ways that would help,
1) less likelihood of being crushed in transit.
2) WAY cheaper shipping. Mail as a letter in a 9x12 envelope. Probably get away with 50-75 cents for 4 or 5 at a time.
BTW, nice job on getting 'em done!
Don't worry; they will ship flat. Even after they are glued and properly folded, it's easy to fold them back down.
I just gotta unfold the two side wings, then I can make the box flat. It's kinda hard to explain, but I tried.

I'll look into First Class Letter vs First Class Parcel, in more detail, as we get closer to completion.
In addition to the choose-your-own-quantity-in-multiples-of-1 order method, I am also going to set up several tiers, or "Packs", if you will, with Fixed Price Shipping, so you all can choose to get X amount shipped in a Flat Rate Envelope, or one of the three Flat Rate Boxes (Small, Medium or Large) or Regional Rate Box A.
It will depend on size, weight, and quantity.
After they're all cut and glued, I'm going to do a lot of testing and experiments with quantity, weights, and packaging, to make sure it's set up to give the best possible shipping rates as I can.
I still, and always will, do custom shipping quotes, upon request, for anything I sell, so if any of my shipping prices seem too high, ya'll can shoot me an e-mail.