The difference is the estimated time of day the carrier will deliver the package. Ground packages are delivered anytime between the hours of 9:00 a.m. and 6:00 p.m. (and sometimes later) to residences, and to business addresses during their normal business hours. Expedited shipments have best effort early delivery times-of-day that vary by service level. Money-back delivery date guarantees are also included for US Postal Service - USPS Priority Mail Express® and United Parcel Service - UPS Next Day Air® domestic US services.