LetterAgent
Mail physical letters by just asking your AI assistant.
Cubic pricing is a USPS system that charges by the size of the box, not the weight, for small dense parcels up to 20 pounds. It beats flat-rate boxes when your item is heavy but tiny. One honest limit first: cubic is US-only, and LetterAgent shipping labels are Canada-only Xpresspost, which prices by weight, so cubic math does not apply to them.
Most shipping prices weight against you: a heavier box costs more. Cubic pricing flips that. USPS multiplies your box's length, width, and height to get its volume in cubic feet, then charges by tier (tenths of a cubic foot). A parcel under 20 pounds in a box under half a cubic foot gets priced on volume alone: the 15-pound paperweight and the 2-pound paperweight in the same small box cost the same.
That is the opposite of dimensional weight, which punishes big, light boxes. Cubic rewards small, heavy ones. The two systems exist side by side because carriers care about two different costs: how much space your box takes on the truck, and how much your box weighs to lift. Cubic is USPS saying: for truly small boxes, space is all that matters.
| Your parcel | Cubic or flat rate? | Why |
|---|---|---|
| A stack of hardcover books in a shoebox-sized box | Cubic | Heavy for its size; volume pricing ignores the weight |
| A bag of coffee or a box of hardware parts | Cubic | Dense and small; flat rate would charge for the weight bracket you are nowhere near |
| A pillow or a bundle of clothing | Flat rate (usually) | Light and bulky; it eats volume tiers fast while weighing nothing |
| A medium flat-rate box filled edge to edge | Flat rate (usually) | The flat-rate price already assumes a full heavy box |
The rule of thumb: if your item is small enough to hold in one hand but heavier than it looks, price it cubic. If it is light for its size, cubic has nothing to offer you.
Three. First, the 20-pound ceiling is hard: a 21-pound dense parcel falls back to weight pricing, where it hurts. Second, the box size limit means anything bigger than half a cubic foot is out; that is roughly a 12-inch cube. Third, cubic is zone-based, so a cross-country cubic parcel can cost meaningfully more than a short hop, while a flat-rate box costs the same anywhere in the US. For a dense parcel going three zones away, cubic usually wins easily; for the same parcel going across the country, run the numbers.
Cubic pricing is a USPS-only concept; Canada Post has no equivalent. Canada Post parcels are priced by weight and dimensions through its own rate structure, and LetterAgent's labels are Canada-only Xpresspost (5 kg max). So if you are shipping inside Canada, the cubic-vs-flat-rate question does not arise: compare Xpresspost against regular parcel rates for your weight and distance instead.
Ready to ship it? Get a shipping label with LetterAgent: describe it to your AI assistant, approve the exact quoted price, and get your label.
LetterAgent is a working name. Prices include print, postage, and our service fee. US prices in USD, Canadian prices in CAD (Canadian prices exclude applicable sales tax, added at checkout). Currently in private testing.
Need help? Ask your assistant: it can check a job's status any time. If it ever can't handle something: hello@getletteragent.com