Subscription box packaging: sizing for the carrier rates that matter
Size a subscription mailer to the smallest inside dimensions that hold the contents with a quarter inch of clearance, then check the outside dimensions against the carrier's dimensional weight divisor and cubic tiers. Keeping the box under half a cubic foot and under 18 inches on any side opens the cheapest bands. E-flute keeps the weight down.
Why the box size sets the postage
Carriers charge on whichever is greater, the actual weight or the dimensional weight. Dimensional weight is length times width times height in inches, divided by a number the carrier sets. The private carriers use 139 for most retail shipments. The postal service uses 166, and only applies it to packages over one cubic foot. A box of 12 by 10 by 8 inches is 960 cubic inches, so at 139 that is about 7 pounds of billable weight even if the contents weigh 2.
The other lever is the postal cubic tiers. A box under half a cubic foot, with no side over 18 inches, can be priced by volume in tiers of a tenth of a cubic foot rather than by weight. A dense box of small heavy items does well here. Carrier rules change, usually each January, so confirm the current divisor and tiers before you lock a size: the postal prices are in USPS Notice 123, and UPS and FedEx publish theirs in their current service guides.
Common subscription sizes and where they land
| Inside size (in) | Outside size approx (in) | Cubic feet | Dim weight at 139 | Cubic tier fits |
|---|---|---|---|---|
| 6 x 6 x 4 | 6.4 x 6.4 x 4.3 | 0.10 | About 1.3 lb | Yes, lowest tier |
| 8 x 6 x 3 | 8.4 x 6.4 x 3.3 | 0.10 | About 1.3 lb | Yes, lowest tier |
| 9 x 7 x 4 | 9.4 x 7.4 x 4.3 | 0.17 | About 2.2 lb | Yes, second tier |
| 10 x 8 x 4 | 10.4 x 8.4 x 4.3 | 0.22 | About 2.7 lb | Yes, third tier |
| 12 x 9 x 4 | 12.4 x 9.4 x 4.3 | 0.29 | About 3.6 lb | Yes, third tier |
| 12 x 10 x 8 | 12.4 x 10.4 x 8.3 | 0.62 | About 7.7 lb | No, over half a cubic foot |
Design the contents to the box, not the box to the contents
The cheapest way to ship is to decide the box first and then curate to fit it. A 10 by 8 by 4 inch subscription box with a die-cut E-flute insert holding four to six items is a practical working size for beauty, snack and stationery boxes. Going up to 12 by 9 by 4 adds an inch of billable weight and a good deal of empty air that then needs crinkle paper to fill. Curating to a fixed box also makes the insert a one-time design rather than a monthly one.
Height is the dimension to fight. Every inch of height is a full slice of volume, and many subscription items are under 3 inches tall. A 10 by 8 by 3 inch box with a snug insert beats a 10 by 8 by 4 inch box with filler on both weight and unboxing. Filler also adds weight, which is the thing you were trying to avoid.
The board and its weight
Subscription mailers here are E-flute corrugated at 32 ECT. A 10 by 8 by 4 inch E-flute mailer weighs about 3.5 ounces flat. The same box in B-flute weighs about 5 ounces and is a quarter inch bigger on the outside, which pushes a borderline box up a tier. E-flute also prints better and folds cleaner. If the contents are heavy, glass jars or canned goods, the answer is not thicker board on the mailer. It is a die-cut corrugated insert inside that stops movement, and, if the total weight passes about 5 pounds, a corrugated shipping box instead of a mailer. Damage comes from things moving, not from board being thin.
An illustrative spec: a coffee subscription
Illustrative spec, not a client order.
Say a coffee subscription ships two 12 ounce bags plus a card and a small bag of samples, 400 boxes a month. The bags lie flat at about 7 by 4.5 by 2 inches each. Side by side that is 9 by 7 by 2 inches of product. We would spec a 9.5 by 7.5 by 2.5 inch inside dimension kraft mailer on E-flute, one color print outside, full color inside liner, with a die-cut E-flute divider so the bags do not slide into each other.
Outside dimensions come in at about 9.9 by 7.9 by 2.8 inches, 0.13 cubic feet, second cubic tier, with a loaded weight of about 1.9 pounds. A stock 10 by 8 by 4 inch box would put the same contents a tier higher and need crinkle paper to stop the rattle. For a mailer printed inside and out, expect 10 to 14 business days after proof.
Check the box before you order 400 of them
Ask for a sample first. A white or printed sample is available on request, with timing confirmed in the quote, so you can pack the real contents, weigh it on a kitchen scale, measure the outside with a tape, and run the numbers through the carrier's calculator. A quarter inch on one side can move a box across a tier. It adds a little time and it is the cheapest packaging decision you will make all year.
Questions people ask
Do carriers measure inside or outside dimensions?
Outside, at the widest points, including any bulge. Our sizes are inside dimensions, so add about 0.4 inch to length and width and 0.3 inch to height on E-flute to get the outside. A box that is packed tight can bow, so leave clearance rather than forcing the lid.
Is a mailer or a poly mailer cheaper to ship?
A poly mailer is lighter and has no dimensional weight at small sizes, so for soft goods like apparel it ships cheaper. For anything that can be crushed or that should arrive presented, a mailer box is the right call and the postage difference is usually small at the sizes above.
What is the 18 inch rule?
The postal cubic pricing only applies to boxes with no side longer than 18 inches and a volume under half a cubic foot. Typical subscription mailer sizes sit well inside both limits. A long narrow box, such as a 20 by 6 by 4 inch poster box, falls out of cubic pricing even though its volume is small.