Priority Mail USPS Flat Rate Envelope. This is one of the best methods to ship since it is much lower than UPS and has no residential address delivery penalty like UPS, DHL or FEDEX. USPS says 2 - 4 days in USA. If it can fit in the envelope it can ship regardless of weight. As an example I have sent one box of the EN95 D_Cell Energizer batteries and a box of EN91 over 5 pounds in the USPS Priority Mail Envelope.