Example scenarios
Square envelopes show up for modern wedding invitations, minimalist greeting cards, and design-forward stationery where the flap makes a square face. Some photo cards ship square to match the print. Small art prints and craft swaps sometimes use square sleeves. In each case the outer shape is the issue for automation, not the sentiment inside. If the face is square and the piece still qualifies as a letter by size and thickness, retail pricing often adds a non-machinable surcharge on top of the ounce price. The calculator mirrors that split so you can plan total dollars and stamps together.
Common mistakes
People sometimes buy only ounce stamps and stop when the ounce ladder looks paid. The surcharge is separate money, not another ounce stamp in the ladder. Another mistake is treating a thick square card like a thin one-ounce letter without checking weight after sealing. A dense card can be two ounces or more. A third mistake is confusing square with merely small. A 5×7 envelope is rectangular; square means equal length and height on the face, or the special handling that goes with that shape in the tool. Assuming all small envelopes mail like standard letters leaves the surcharge out of the budget.
How USPS calculates postage
Letter postage starts with whole ounces rounded up from actual weight. Size must fit letter limits for length, height, thickness, and weight. Machinable letters run through sorting equipment efficiently. Square envelopes, rigid pieces, and lumpy contents often count as non-machinable for letter pricing. When non-machinable applies, USPS adds a surcharge on top of the ounce total for qualifying letter mail. This tool shows the surcharge as its own line next to the ounce breakdown. If dimensions or thickness fail letter tests, the piece may be a large envelope instead, and letter stamp logic stops applying.
How the surcharge works on square mail
The non-machinable surcharge is a fixed dollar add-on in this model, not a substitute for an additional ounce. You still pay the first ounce at the letter rate and each extra ounce at the additional-ounce steps. The surcharge sits on top as its own postage. In stamps, you might use a mix of denominations to reach the total, or buy postage online where the exact amount prints on the label. The breakdown in the calculator lists first ounce, additional ounces, surcharge, and total so you can match retail expectations. If you leave the square option off when the envelope is truly square, your estimate will read low. If the envelope is square but too thick or too large for a letter, you will see the large-envelope message instead of letter pricing. For rigid stock or lumpy contents, see the non-machinable mail guide and compare with rectangular envelopes on the 1 oz and 5×7 pages when the mail is not square.
Before you mail
Weigh and measure after packing. If the mail is valuable or time sensitive, confirm class at retail when you are unsure. Metered or online postage with a barcode reduces guesswork at the mailbox.