The U.S. Postal Service determines the cost of First-Class Mail by the weight of the letter. Inadequate postage results in letters being returned to the sender. Flat-rate envelopes and boxes follow a different method of mailing with a specified price for the size of the envelope or package with a maximum weight limit. Other parcels define cost based on size, shape and weight of the package.
TL;DR (Too Long; Didn't Read)
U.S. Postal Service representatives are trained to help consumers choose the least expensive option with the most expedited shipping to deliver as much value as possible.
First-Class Mail Postage
The current First-Class Mail postage stamp rate for standard letters is 50 cents for the first ounce. Every additional ounce is 21 cents. Letters that consist of four or five standard stock pages of paper in a legal-sized envelope weigh approximately an ounce. When in doubt, use a calibrated mail scale. The 50-cent rate is higher for postage stamps than for metered mail which has a rate of 47 cents for the first ounce.
Larger items cost more to mail. A large first-class envelope starts at one dollar for the first 1 ounce. Additional ounces remain 21 cents per ounce. Postcards cost 35 cents for postcards with dimensions up to six inches by 4.25 inches. Postcards larger than this, up to 11.5 inches by 6.125 inches cost 50 cents.
Flat-Rate Envelopes and Boxes
Flat-rate envelopes and boxes allow senders to pack as much as they can fit in the packaging up to a 70-pound weight limit. This weight limit is for all sizes of flat-rate items, a catch-all limit that is overly generous. Costs start at $6.70 for small priority mail flat-rate envelopes and go up for express delivery options. Flat-rate boxes range from $7.20 to $18.90 as the size increases.
Shipping Nonstandard Packages
Anything that isn't a flat-rate box is considered a nonstandard package and is shipped via Parcel Select Ground service. Costs are based on shipping weight in pounds. Prices start at $6.55 for the first pound and increase based on desired delivery speed.
Media mail is used for boxes with books, CDs and DVD items. Media mail is used for less time-sensitive mailings. Media mail is less expensive per pound and is delivered with less urgency. Media mail is calculated based on each pound beginning at $2.66 for the first pound. If a media mail package is 10 pounds, the cost is $7.25.
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