ALERT for indie makers and publishers: On July 1, USPS rates will increase from a little to a lot for packages. Make sure you update the fees on your websites and e-commerce platforms!
Media Mail is 50–60¢ more expensive (overall, not per pound) for starters. Ground Advantage jumps a couple bucks or more as part of the base price.
https://pe.usps.com/PriceChange/Index
Use Pirate Ship et al for discounted rates—going to USPS.com will get you a better rate than the PO counter, but 3rd parties are better yet.