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. Please see the table below for the available Priority Mail packaging options.
Otherwise, you may use any packaging materials you may have laying around the house to ship. As long as the packaging is intact, 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.