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When ShipMagic asks a carrier for a rate, the carrier needs to know what it’s quoting for. The packing algorithm decides how your cart’s items are distributed into parcels before that question is asked. Get this right and live rates match what you’re actually charged. Get it wrong and every quote is off.

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Packaging box distribution

Every live carrier rate has a Packaging box distribution setting with four options:
All four options except None need you to define at least one box. By items volume additionally needs accurate product dimensions — see Adding Dimensions to Products. Without them the algorithm has nothing to pack.

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Defining boxes

Once you pick a distribution method other than None, a box table appears. Each box takes: Add as many box types as you use with Add another box.
Include the empty box weight. It’s easy to leave at zero, but a heavy carton on a light order can shift the parcel into the next weight bracket and make every quote too low.

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How the 3D algorithm works

By items volume is the most sophisticated option. It uses 3D bin packing: each item is treated as a rectangular solid with a width, height, depth, and weight, and the algorithm fills your boxes by volume, tracking remaining space and accumulated weight as it goes. When one box is full it opens another of the same or the next suitable type, until every item is placed. ShipMagic then asks the carrier for a rate on the resulting set of parcels.
The algorithm packs by volume, not by simulating physical orientation. It won’t detect that a long item cannot fit diagonally into a box whose volume is technically sufficient. For awkward-shaped goods, define a box that genuinely fits them and use By items count.

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Choosing a method

None - Use product dimensions suits stores where each product ships in its own packaging — furniture, appliances, anything already boxed by the manufacturer. By items count is the simplest to reason about and the easiest to keep accurate. If you ship a fairly uniform catalogue in one or two carton sizes, start here. By weight suits dense goods where weight, not space, is the binding constraint. By items volume is the most accurate for mixed carts of varied sizes, and the most demanding — every product needs correct dimensions.
Start with By items count and move to By items volume once your dimension data is complete and trustworthy. An accurate simple model beats an inaccurate sophisticated one.

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Pallets

For freight-scale orders, ShipMagic also supports pallet configuration — see the pallet fields on carriers that support them, such as Direct Freight Express.

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Verifying your setup

Live carrier rates are hard to eyeball. Test with real carts:
  1. Add a cart you know the true shipping cost for
  2. Check the rate at checkout against what the carrier actually charges you
  3. Try a cart that should need two boxes, and confirm the rate rises appropriately
Always set a Default rate on live carrier rates. If the carrier doesn’t respond, that fallback is the only thing standing between your customer and a checkout with no shipping options.

Adding dimensions

Populate length, width, and height across your catalogue.

USPS

Connect a USPS account.

FedEx

Connect a FedEx account.

Rates not showing

When a carrier rate fails to appear.