Spent 40 minutes at the post office and learned a lot about the shipping methods...
They have boxes (and labels and tape) which are FREE. They are FLAT RATE boxes, meaning that up to 20 pounds, you pay only one price for whatever you can fit inside. They run around $5 for a VHS-sized box, $10.35 for a medium-sized box, and they have a larger one (I forget the $$$).
The FLAT RATE boxes are best for when you are shipping a lot of stuff (or weight) over a long distance.
They also have boxes that are just weighed for a price. If you are sending only a few items (lightweight) and it's a short distance, this is the best route.
You can also use your own boxes for that second method.
Visit the site listed in OP for the USPS for more info on shipping costs by size.