How Do I Send Priority Mail (Flat Rate Mailing Envelope)? May 15, 2014 · 1. To prepare a flat rate envelope at home, you will need to use a credit card to prepay the Priority Mail flat fee and to print out a postage label plus a shipping label. If you do not have self-sticking labels, use paper labels and attach them to the envelope with clear shipping tape. You can schedule a pick up by the USPS.