Whatever you do, avoid supplyhut. They are usually the cheapest, but there's a reason for that. I'm convinced they sell factory rejects. Stay away. I prefer the boxery as they sell good quality stuff More on reddit.com

Buy a conventional padded mailer (not bubble wrap). I buy foam board and cut it to size; the foam board is what protects it from creasing or bending. Either two thin front and back, or thicker across the front. I also like to cover the surface with wax paper or parchment paper. I do this for matted drawings. More on reddit.com

I originally started buying photo mailers from Staples when I first started needing rigid mailers. The quality was good but they were a bit pricey. They aren't a bad option if you're just starting out and not sure what kind of sales volume to expect. They usually come in packs of 4. I outgrew those pretty quick and switched to the Quality Park Extra Rigid mailers on Amazon. The quality of these were good but the price just recently doubled (and they were overpriced to begin with) so I've been looking for other options. Most of the "rigid" mailers on Amazon are garbage tbh. Many things that are labeled rigid mailers are the equivalent of 65lb cardstock and bend super easily. I even once ordered custom printed ones from Vista Print because I wasn't having any luck on Amazon. Those ones were super flimsy as well and arrived in terrible condition. I actually just 2 days ago sucked it up and ordered the Stayflat envelopes from Uline. Like you I didn't love how much they charge for shipping so had been avoiding them up until now but with Quality Park doubling their price I really needed to look elsewhere. That being said it was a much better deal than ordering from Amazon and the quality is really good (they are actually rigid and not flimsy at all). It was $66 CAD plus $35 shipping (I also purchased 250 bubble mailers that shipped with the same $35 shipping) for 100 rigid envelopes. I was paying $36 CAD for 25 envelope packs on Amazon so even at $101 for 100 from Uline (including shipping) it was cheaper than the $144 it would have cost me for the same rigid envelopes on Amazon. If you're just starting out I would probably go with the Staples photo mailers and if you find you're going to through them too quickly I would switch to Uline. More on reddit.com

For my 6" x 9" hardcover book, I use ULINE S-11215 mailers and two 12" x 12" squares of Duck bubble wrap . If you ship via USPS Media Mail, you get a tracking number, and it shows up in 2-8 business days. For my book, which is about 2 pounds, the whole thing costs $7.22 to pack and ship -- $1.61 for the mailer and bubble wrap, and $5.61 for the USPS shipping (after the recent rate increase). This is kind of a premium solution, though :) It's thick (1-2 layers of cardboard and bubble wrap on every side) and waterproof if you seal it well, but it's expensive and time-consuming. I do it because mostly the only people ordering my hardcover edition are friends and family -- strangers buy the paperback or Kindle versions! I've noticed that online booksellers who drop-ship my hardcover directly from IngramSpark use "UPS Mail Innovations" (where UPS hands it over to USPS for economy shipping), which has a tracking number but seems somehow cheaper than USPS Media Mail. They ship the book in a stiff paper package that gum-seals around all four sides in a wide flange, kind of like a giant ravioli. The envelope has a thin layer of plastic on the inside, not bubble wrap but a slight bit of extra protection. Based on the price they're charging for the book, my guess is that this costs them maybe $6.00 all up. But this option seems to be available only to large customers. For reference, if I were to order a single copy of the same book from IngramSpark myself, and have them ship it to my customer, they would charge me $9.69 for the shipping, though this presumably includes some handling charge as well. They charge $17.39 to ship a carton of 12 of the books, which works out to $1.45 per book. So I order them sent to me by the carton, then ship them to customers myself :) More on reddit.com