Buyers pay for the shipping cost, not for the shipping supplies. USPS will give you free packaging for Priority Mail shipments, unless the item is oversized.
You can use ANY packaging materials you may have laying around the house to ship. As long as the package is secure, then you're good to go!
Please remember, if you're shipping via USPS Priority Mail, you must use Priority Mail packaging. Similarly, if you're shipping via UPS, you cannot use USPS packaging.