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If you sell autographed copies of your print editions, as I do, you can save money by using PayPal instead of Click'n'Ship. There's a more or less hidden portal to get to the service without actually selling the book through PayPal (though I do that as well): https://www.paypal.com/signin/?returnUri=https%3a//www.paypal.com/cgi-bin/webscr&state=%3fcmd%3d_ship-nowClick'n'Ship charges $6.95 for a Priority Mail envelope, and does not sell Media Mail postage. PayPay charges a dollar less for Priority Mail, and it does offer Media Mail and First Class Small Package service.
For US domestic shipping, I recommend purchasing a media mail envelope from the post office. The $2 envelope will accommodate a 6x9 hardcover up to a 1.5" spine. The media mail rate to anywhere in the continental US is between $3 - $3.50. That comes to about $5.50. Less if you purchase the same envelope from Amazon in bulk.