If it is too thick to be mailed as a letter at all (i.e. over 15 cm/6 in), they will refuse to accept it, and tell you to send it as a parcel.
For a mailpiece to be eligible for First-Class Mail letter rates, it must be at least 3-1/2 inches by 5 inches by 0.007-inch thick, and no more than 6-1/8 inches by 11–1/2 inches by 1/4-inch thick. If the mailpiece falls within these dimensions, it is classified as a letter. Follow these easy steps for letters: 1.
If your mail piece is too long, wide, thick, or heavy in any single dimension then your mail piece does not qualify for a particular shape. If your mail does not fall into any of these categories (too short or too narrow), it is likely undeliverable as First Class Mail; your mail piece may qualify as a package.
The maximum size for an envelope to be considered a “flat" is 15” long by 12” high by 3/4” thick. If the envelope exceeds any of those measurements, it no longer considered a “large envelope” or “flat”. Instead, it is treated as a package or a “parcel” and subject to much higher postage rates.
Customers must affix additional postage when mailing letters weighing in excess of 1 ounce and/or letters subject to the nonmachinable surcharge or mailpieces subject to another rate of postage (e.g., large envelopes or packages).
(There are five different postage rates for letters, depending on weight, size, and thickness.) If it is too thick to be mailed as a letter at all (i.e. over 15 cm/6 in), they will refuse to accept it, and tell you to send it as a parcel.
For any domestic mail piece that weighs more than one ounce, you must include additional postage along with your Forever Stamp to ensure USPS delivery. Also, you may purchase higher denomination postage stamps for heavier postage at the Post Office™. Avoid placing two Forever Stamps on a mail piece for heavier mail.
To be eligible for mailing at the price for letters, a letter must be: Rectangular. At least 3½ inches high x 5 inches long x 0.007 inch thick. No more than 6⅛ inches high x 11½ inches long x ¼ inch thick.
Business Mail 101
The words large envelopes and flats are used interchangeably. Whatever you call them, flats must: Have one dimension that is greater than 6-1/8 inches high OR 11-½ inches long OR ¼ inch thick. Be no more than 12 inches high x 15 inches long x ¾ inch thick.
The maximum size for letters did not change: 11-1/2" x 6-1/8" x 1/4" thick. When a piece of First-Class Mail exceeds any one of the maximum measurements of a letter, it is classified and priced as a large envelope (flat). The maximum weight for letter-size pieces is 3.5 ounces.
Small letters include enveloped mail, lettersheets and unenclosed postcards. A small letter is: • no larger than 130mm x 240mm • no thicker than 5mm • no heavier than 250g. To avoid small letters being trapped in machinery or other letters, small letters should exceed 88mm x 138mm.
Usually, First-Class Mail envelopes are of 1 oz. The rates also begin at 1 oz pricing, which is $0.60—the cost of one Forever stamp! The maximum weight of letters for postage must not exceed 3.5 oz. Please remember to measure your envelopes after you stuff them to get the accurate weight for postage.
Letters, large or thick envelopes, tubes, and packages containing mailable items can be sent using Priority Mail Express. This money- back guaranteed service includes tracking and insurance up to $100. Additional merchandise insurance up to $5,000 may be purchased.
If your envelope isn't flat, it won't fit in automated processing equipment and you will be charged more. Large envelopes. This includes 9x12, legal, and manila envelopes, Two stamps are needed for the first ounce, and postage will be an additional $0.20 cents after. International mail.
Standard letters. If your letter is a standard-sized envelope and weighs 3.5 ounces or less, you will need one stamp. Large envelopes. This includes 9x12, legal, and manila envelopes, Two stamps are needed for the first ounce, and postage will be an additional $0.20 cents after.
Addressing a large envelope (also known as a “flat”) is the same as addressing a typical letter. Put the recipient's address in the center of the envelope and the return address in the top left corner.
Different postage prices apply to postcards, letters, large envelopes (flats), and parcels. In general, the bigger your mailpiece, the more it will cost to mail.
Non-machinable mails are letters that are not able to be processed by postal machines. These letters may be irregular in size or shape, or they may be too thick, rigid, or have unusual contents. Given that non-machinable mail has to be hand processed, USPS requires extra postage to take this extra work into account.
Speaking of odd shapes, don't mail bulky, odd-shaped things like pens or bottle caps in regular letter-size envelopes. Not only will you pay more in postage, but these items are likely to poke through the envelope, fall out, and be lost -- they could even damage postal equipment or hurt someone.
*Saving tip* – Fold your A4 documents in half to fit them into C5 envelopes and if they are no more than 5mm thick and under 100 grams they qualify as a standard letter.
T-shirts are some of the most common items and are fairly light, typically weighing between 150–200g. If folded up properly with a piece of cardboard to stiffen the packet, you could send t-shirts as large letters. Light jackets and jeans are a little heavier and require a little more room, so small parcels will do.
A padded or stiff envelope is a must if you'll be mailing small items—including keys.
To be eligible for mailing at the price for letters, a piece must be: Rectangular. At least 3-1/2 inches high x 5 inches long x 0.007 inch thick. No more than 6-1/8 inches high x 11-1/2 inches long x 1/4 inch thick.
So, a forever First-Class stamp only buys you enough postage for a one (1 oz) ounce letter. Most letters and postcards weigh less than 1 ounce, though, and therefore require only one Forever Stamp. However, if your letter weighs more than that, you'll need to pay for additional ounce stamps…and we get into that below.
BTW, you are never required to prepare mail as bulk mail, no matter how many pieces you have. USPS is perfectly happy for you to put First Class postage on every piece you mail, and they'll take care of the rest.