Small online sellers lose margin on shipping in three places: boxes that are billed on size instead of weight, packaging nobody counted, and shipping charges that don't cover the label. All three are fixable with a scale, a tape measure and a short spreadsheet.
Check what a box will be billed at with the free Dimensional Weight Calculator.
Know your billable weight
Many carriers charge the higher of a parcel's actual weight and its dimensional weight: length × width × height divided by a divisor. A common US domestic divisor is 139 cubic inches per pound, but carriers and services differ, so check the current figure. A 14 × 10 × 8 inch box is 1,120 cubic inches; at 139 that's about 8.06, rounded up to 9 lb, even if the pillow inside weighs 4.2 lb.
Use the smallest safe box
Dimensional weight punishes empty space. Stock three or four box sizes that fit your products snugly, plus padded mailers for anything flat or unbreakable. Re-measure your best sellers: dropping from a 12 × 10 × 6 to an 8 × 6 × 4 box can cut the billable weight from 6 lb to 2 lb.
Count packaging as a cost per order
A box, void fill, tape, a label, an insert and four minutes of packing time add up. $0.85 for the box, $0.30 fill, $0.15 tape and four minutes at $18 an hour is $2.50 an order before the label. Multiply by your monthly orders and it is often larger than sellers expect. The free Packaging Cost per Order Calculator works it out.
Compare carriers for your real parcels
Take your three best-selling products, note their billable weights and the zones you ship to most, and price each on every carrier's own calculator. The cheapest carrier for a light mailer is often not the cheapest for a bulky box.
Check what you charge against what you pay
Log shipping charged and label cost for a month of orders. If shipping is losing money on average, either raise the shipping charge, build part of it into the product price, or set a free-shipping threshold above your average order so more orders carry it.
Keep a damage log
Write down every breakage with the packaging used. Two or three entries usually show which product needs more cushioning. The Shipping Cost & Packaging Workbook does the billable-weight, packaging and charged-vs-paid maths; the Printable Packing & Shipping Checklists set up the packing station.
This guide is general information. Carrier divisors, rates and surcharges change; confirm with each carrier.