The United States Postal Service offers a few different regular mail delivery services, including Priority Mail Express, Priority Mail, First Class Mail, and USPS Retail Ground. USPS Retail Ground, the most affordable mail service, usually takes between 2-8 business days.
First Class mail is used for letters and other small objects that fit into common envelope sizes. The maximum weight for First Class mail is 13 ounces. Mail can be sent internationally using First Class mail, but the delivery time varies based upon the location. Within the United States, First Class usually takes 1-3 days.
Priority Mail allows items (up to 70 lbs) that would otherwise have to be priced by weight to be sent to their destinations in flat rate packages of various sizes. Priority Mail generally takes between 1-3 days.
Priority Mail Express is meant for packages that need to reach their destination overnight. Packages up to 70 lbs are guaranteed to reach their destination by the next morning if mailed with Priority Mail Express, or the customer will receive a refund.