Fit rules out weight
A light item that does not fit inside the Large box's 12 x 11.75 x 5.5 in frame cannot use flat rate no matter how little it weighs. The box shape is the first filter, not the last one.
USPS pricing
Flat rate shipping to Hawaii: same box, same price, any island
USPS prices a flat rate box by the box, not the mile: one price for anywhere in the United States, at any weight up to 70 lb. That claim matters more in Hawaii than almost anywhere else, because Hawaii is usually the most expensive zone on a weight-based quote. We tested it on 2026-08-25 with two boxes to Honolulu, one sized to the Medium Flat Rate Box and one to the Large.
USPS pricing, read 2026-08-25Live quotes, same dayHonolulu, HI lane
One price, regardless of the ocean
Flat rate does not know the difference between a neighboring state and an island 2,500 miles offshore. That is the entire trick.
What the flat rate menu actually costs
These are USPS's own published prices, read from usps.com's Priority Mail page on 2026-08-25. USPS states the price is "the same price for mailpieces weighing up to 70 lbs going anywhere in the U.S.," a sentence that applies to Honolulu exactly as it applies to Trenton.
| Product | Retail | Commercial (online) |
|---|---|---|
| Flat Rate Envelope | $12.90 | $11.12 |
| Padded Flat Rate Envelope | $14.00 | $11.99 |
| Small Flat Rate Box | $13.65 | $12.10 |
| Medium Flat Rate Box | $24.80 | $21.17 |
| Large Flat Rate Box | $34.00 | $31.00 |
None of these prices move for Hawaii, Alaska, Guam or any other state or territory. The box price is the box price, and the only ceiling is 70 lb and whatever physically fits inside it.
Two boxes, tested against a live Honolulu quote
A published rule is easy to state and easy to ignore. We built two boxes to the inside dimensions USPS publishes for the Medium and Large Flat Rate Boxes — 11 x 8-1/2 x 5-1/2 in for the top-loading Medium, 12 x 11-3/4 x 5-1/2 in for the Large — loaded them to a realistic dense weight, and ran both through our own checkout on the same lane: Berkeley Heights, NJ 07922 to Honolulu, HI 96813, quoted 2026-08-25. The weight-based figure below is USPS Priority Mail, the fastest-tier product that fairly compares to flat rate's speed.
| Box built to fit | Weight | Priority Mail by weight | Flat rate box (online) | Flat rate saves |
|---|---|---|---|---|
| Medium Flat Rate Box 11 x 8.5 x 5.5 in | 11 lb | $24.56 | $21.17 | $3.39 |
| Large Flat Rate Box 12 x 11.75 x 5.5 in | 25 lb | $129.49 | $31.00 | $98.49 |
The gap at 25 lb is not a rounding difference; it is the entire logic of Hawaii's zone pricing running into a product that ignores zones. Both boxes quoted with USPS Ground Advantage available too, at $17.66 and $92.27 respectively, but that tier carried a 14-15 business day window on this lane against Priority Mail's 3-4 days, so it is not the fair comparison for a flat rate product that ships at Priority speed. We tested these two weights only; a lighter box may still favor weight-based shipping, so run your own numbers before assuming flat rate wins by default.
Where flat rate stops helping
Light items rarely need the trick
Flat rate wins by absorbing a distance penalty that weight-based pricing charges in full. A light box to Hawaii already starts from a lower weight-based number, so the flat rate box has less penalty to absorb and less to win by.
70 lb is a hard wall
Past 70 lb, or past what fits inside the Large box, the flat rate product is not on the menu at all, and the shipment prices by weight and dimension like anything else.
Where these facts come from
- USPS, Priority Mail — current flat rate envelope and box prices, and the same-price-anywhere-in-the-U.S. statement. Read 2026-08-25.
USPS revises this page without notice. Treat the prices above as what usps.com stated on the date shown, and the weight-based Priority Mail figures as our own checkout's quotes from the same day.
Common questions
Does a USPS flat rate box cost more to ship to Hawaii?
No. USPS states on its Priority Mail page, read 2026-08-25, that a flat rate box is "the same price for mailpieces weighing up to 70 lbs going anywhere in the U.S." Hawaii is a US state, so the box that costs $31.00 online to a neighboring state costs the same $31.00 to Honolulu.
Is flat rate actually cheaper than paying by weight to Hawaii?
On the two boxes we tested to Honolulu on 2026-08-25, yes, and the gap grows with weight. A dense 25 lb box sized for the Large Flat Rate Box quoted $129.49 by weight through our checkout against $31.00 for the box online, a $98.49 difference. An 11 lb box sized for the Medium box quoted $24.56 by weight against $21.17 online, a smaller but real $3.39 gap.
Can I buy USPS flat rate postage through SMKlog?
No. Our checkout prices your actual box by its measured size, weight and destination, which is a different product from the fixed-price flat rate boxes USPS sells at the counter or on usps.com. The flat rate figures on this page are USPS's own published prices, listed so you can compare both approaches before paying anywhere.
What if my box is too big for the largest flat rate size?
Then flat rate is not an option regardless of weight. The Large Flat Rate Box tops out at 12 x 11-3/4 x 5-1/2 in, and anything past that ships by weight and dimension instead. Run the actual box through the calculator to see the weight-based price to your Hawaii ZIP.
Does the 70 lb flat rate ceiling really apply at any weight up to that?
Yes, as long as the contents fit inside the box. USPS prices flat rate by the box, not the scale, so a 68 lb box of dense parts and a 3 lb box of the same size cost the same flat rate price. Weight only matters again once it exceeds 70 lb, at which point USPS stops selling the flat rate product for that piece entirely.