How to choose a shipping box size without guessing
Box selection tends to be a habit rather than a decision. Somebody chose a set of sizes once, and every order since has been fitted into whichever of them looks about right. The cost of that habit is invisible, recurring, and larger than most people expect.
Why the default box costs you twice
An oversized box costs you on both sides of the transaction. You pay for the void fill that stops the contents moving, and you pay the carrier for dimensional weight — a charge calculated from the size of the box rather than what is in it. Neither cost appears as a line item labelled "wrong box".
A method that takes ten minutes
Start by writing down what you actually ship. Not your whole catalogue — the ten or fifteen products that make up the bulk of your orders, plus the three or four combinations that recur most often.
Then take each of your current box sizes and check what genuinely fits. Not what you think fits: what the geometry allows, with the products in the orientations they can actually be packed in.
Finally, run the same test against the next size down. This is the step almost nobody does, and it is the one that pays. If a common order fits in a smaller box, every future instance of that order has been overpaying.
Fewer sizes, chosen deliberately
The instinct after an exercise like this is to add more box sizes to cover every case precisely. Resist it. Every extra size is more inventory to hold, more shelf space at the packing station, and one more decision for the person packing. A small set of sizes chosen deliberately beats a large set chosen by accumulation.
The goal is not a perfect box for every order. It is knowing, rather than guessing, which box each order belongs in.