USPS Mail Classes
First Class Mail
First-class mail is material wholly or partially handwritten (including carbons), postcards, completed forms, statements, invoices, and typewritten or computer processed correspondence. Abnormal objects (pens, etc.) cannot be sent in regular sized envelopes and need to be sent in padded envelopes.
Up to and including 13 ounces. See "Priority Mail" for pieces over 13 ounces.
Minimum size is 3-1/2"x 5" for envelopes, cards, and self-mailers. Cards larger than 4-1/4"x 6" require first-class postage. CAUTION: orientation of the address label may affect eligibility for mailing.
Additional postage is required for first-class mail weighing one ounce or less which exceeds 11-1/2" (L), 6-1/8" (H) or 1/4" (D). The placement of the address determines which side of the piece is the length; therefore, if the piece is addressed such that the address is read across the short side of the piece, the postage surcharge may apply.
Priority mail consists of all first-class mail weighing over 13 ounces and up to 70 pounds. In addition, mail weighing less than 13 ounces may be sent as priority mail, provided that the postage for the minimum priority rate (14 ounces) is paid. The USPS targets a two-day delivery for priority mail, however two-day delivery is not guaranteed. Any type of mail (e.g. books, printed matter, etc.) may be sent as priority mail when expediency is necessary.
Parcel post is printed matter, merchandise, or other mailable material that weighs more than 16 ounces. Parcels should be marked "Parcel Post."
Essentially "Media Mail" is used for sending books (at least 8 pages, permanently bound) and educational materials including manuscripts, catalogs, films, printed sheet music, educational reference charts, computer readable media, specimens, and objective test materials. This rate is less expensive than regular standard mail, and each piece should be marked "Media Mail."