The first category, letters, includes envelopes under .25" thick, 11.5" long, and 6.125" high. (Minimal dimensions are 3.5" by 5".) These are currently priced at $.45 for the first ounce, then $.20 for each additional ounce up to 3.5 oz. If a letter is encased in plastic or contains something rigid, such as a key, paper clip, or credit card, or the address is not parallel to the longest side, it also incurs an additional $.20 "non-machinable" surcharge because it must be processed by hand rather than by the electronic sorting machine. Square envelopes also incur this charge because their sides are of equal length.
Envelopes up to 13 oz. (including letters over 3.5 oz.) and between .25" and .75" in thickness, 11.5" and 15" long, and 6.125" and 12" high are classified as large envelopes/flats. The base postage for these is usually double the letter rate (two letter stamps), and additional ounces are usually priced the same as for letters. If a large envelope contains something rigid, or there is more than a .25" variation in its thickness, it is classified as a parcel rather than just receiving a $.20 surcharge.
Parcels are mail under 13 oz. which do not meet either the letter or large envelope criteria. Base pricing for these varies; currently it is $1.95.
Mail which is over 13 oz. may not be shipped First Class and also may not be dropped in a mailbox or handed to a carrier. Instead, it must bear a postage meter strip identifying its point of origin. These