If you are primarily selling clothing and/or other soft items, we recommend using poly mailer bags such as those found here or at an office supply store. Other supplies you may need include packaging tape for the boxes and for taping the label on (if you print the label at home), and tissue paper for wrapping the item(s) before you package them up.
If you are shipping large or heavy items, consider using USPS Priority Flat Rate envelopes or boxes. The mailers are free* and can be delivered to you for free by going here, or you can pick them up at your local Post Office.
*Important Note: Priority Flat Rate envelopes/boxes can only be used with Priority/Flat-Rate postage. If you do not specifically purchase the accompanying Priority Flat-Rate postage, you may not use anything labeled Priority to ship. USPS will reject or add additional charges packages with First-Class postage in a Priority container.