They are good, but you need to use the cardboard sandwich. People stick a book just with a bag and board ... not good .. and even worse, they TAPE that book to the inside of the mailer, which is almost impossible to undo without bending the heck out of the book. I am not sure how anyone would view them as "flimsy"... if you are using a sandwich inside. A single book is 11-12 ounces, so you can often get it out first class for under $4.50. I bought a bunch of them at once and I think I paid 60 cent each. So I only lose a little bit of $ shipping first class with them at $5, and if sent media, you break even, maybe make a little. If you want to be really good, make the sandwich, wrap it in saran wrap to help protect from any moisture, And you should probably tape the edges of the gemini too.