When shipping printed or recorded material, such as books, CDs, or DVDs, you will generally save money shipping via Media Mail instead of Priority Mail or First Class. Bring your package to the post office and ask the clerk for the comparative postage costs.
Media Mail is best used for packages weighing more than a pound, and it will take longer for delivery. Using ZIP+4 can speed up Media Mail for no extra charge.
If the package weighs less than a pound, First Class might actually be cheaper, or close enough to not justify the slower delivery of Media Mail.