Does the USPS charge for boxes?

Excuse me, I've been looking into sending a package through the United States Postal Service, and I was wondering if they charge for the boxes that they provide for shipping? I've heard some services include packaging materials in their shipping costs, but I'm not entirely sure about the USPS's policy. I'd appreciate it if you could clarify this for me, as I want to make sure I'm prepared with all the necessary items before heading to the post office. Thank you!