Cheapest Shipping by Carrier — September 2026 Benchmark Issue | DimMath
A fixed monthly rerun of DimMath’s benchmark package set across UPS, FedEx, USPS Ground Advantage, and USPS Cubic using verified published 2026 rate tables. Not live carrier quotes. Not negotiated account rates.
Hub: Which Shipping Carrier Is Cheapest? Updated Monthly Analysis. Prior issue: August 2026. Files: latest CSV, September JSON.
01 — Executive summary
September 2026 reruns six named cartons at zones 2, 5, and 8, once as residential and once as business (36 cells). USPS Ground Advantage and Cubic dollars come from Notice 123 commercial tables (effective 12 July 2026). UPS Daily DIM uses divisor 139 with ceiling rounding. UPS residential surcharge is the published $6.60. FedEx residential $6.45 is DimMath 2026 article context. UPS and FedEx base dollars may be modeled estimates.
Pattern that still holds:
- Residential light mailer (10 × 7 × 1 in, 12 oz) and small box (9 × 6 × 4 in, 1 lb): USPS Ground Advantage.
- Dense cubic candidate (10 × 8 × 6 in, 5 lb): USPS Cubic when eligible.
- Shoebox (14 × 10 × 8 in, 3 lb): USPS still competitive on residential; business cells tighten the spread.
- DIM-risk carton (18 × 14 × 12 in, 8 lb) and heavy parcel (16 × 12 × 10 in, 20 lb): private-carrier modeled estimates plus DIM, especially zone 8 business.
Open any cell in the Carrier Savings Engine.
02 — What changed versus August 2026
No new published base-rate table was logged in September. This issue is a rerun against the current verified config. USPS Notice 123, Cubic 22-inch / 20 lb gates, UPS $6.60 residential, and UPS Daily DIM 139 are unchanged from the August archive.
The only calendar note: the 2026 UPS Rate & Service Guide Daily PDF identified 7 September 2026 remains the DIM-rounding source already wired into the calculators. It did not introduce a new DimMath commercial dollar table for this benchmark.
If winners look the same as August, that is expected. We do not invent a “this month’s shock” when the tables did not move.
03 — Benchmark matrix
Scenarios (inches and pounds, imperial canonical):
- Light mailer — 10 × 7 × 1, 12 oz (0.75 lb)
- Small box — 9 × 6 × 4, 1 lb
- Dense cubic candidate — 10 × 8 × 6, 5 lb
- Shoebox profile — 14 × 10 × 8, 3 lb
- DIM-risk carton — 18 × 14 × 12, 8 lb
- Heavy parcel — 16 × 12 × 10, 20 lb
Each row in Dataset 07 stores UPS estimate, FedEx estimate, USPS GA estimate, USPS Cubic or null, cubic_eligible, dim_applies, surcharge_flag, winner, runner_up, and spread.
Preview the Zone 5 residential slice on Open Rate Data — Dataset 07. Do not treat modeled UPS/FedEx cells as invoice-ready.
04 — Named scenario breakdowns
Light mailer. Volume is far under 1,728 in³, so USPS bills actual weight. UPS Daily still computes DIM. Residential surcharge makes UPS/FedEx uncompetitive. Cubic may be eligible on volume but is not always cheaper than the light GA weight cell.
Small box. Still a USPS residential story. Cubic eligibility is real; compare GA vs Cubic in the download rather than assuming Cubic always wins at 1 lb.
Dense cubic candidate. 480 in³ ≈ 0.28 ft³ after cubic quarter-inch rounding — GA Cubic tier territory, longest side 10 in, weight 5 lb. This is the profile Cubic was built for.
Shoebox profile. 14 × 10 × 8 is still under 22 in. Watch DIM on the private-carrier path. Residential surcharge can still keep USPS ahead until zone and DIM stack.
DIM-risk carton. 18 × 14 × 12 is 3,024 in³ billed on ceiling inches — over 1 cubic foot. USPS DIM 139 applies. Cubic fails the 1.0 ft³ GA cap. surcharge_flag may include USPS nonstandard volume (> 2 ft³ is 3,456 in³; this carton is under that volume gate but over the DIM gate).
Heavy parcel. 20 lb actual, 16 × 12 × 10. Cubic is eligible on size/weight/volume (1,920 in³ < 1,728? 16×12×10 = 1,920 in³ > 1,728 — Cubic uses round-down to quarter inch: 16×12×10 = 1,920 / 1728 ≈ 1.11 ft³, so GA Cubic is not eligible over 1.0 ft³). Winner then sits between GA DIM/weight and UPS/FedEx modeled billable weight.
05 — Residential surcharge monitor
UPS $6.60 per residential package (published, effective 6 July 2026). FedEx $6.45 in DimMath’s 2026 article context. USPS $0. Business cells omit both add-ons. That single switch is why a “UPS is cheaper on the base” story can reverse on DTC residential volume.
This benchmark does not look up Delivery Area Surcharge by ZIP.
06 — Cubic watch
Gates unchanged: longest side ≤ 22 in, actual weight ≤ 20 lb, GA Cubic ≤ 1.0 ft³, dimensions rounded down to the nearest 1/4 in for volume. Ineligible rows store null Cubic rather than a fake rate. Check a live carton on the USPS Cubic Calculator.
07 — DIM / threshold watch
UPS Daily: round each side up, volume ÷ 139, bill max(actual, DIM) with no 1 ft³ gate. USPS: DIM only when billed volume > 1,728 in³. FedEx in this stack uses the same ceiling model as UPS for billable weight and is not independently verified from a 2026 FedEx Rate Guide PDF. Additional Handling dollar amounts stay unlogged; flags still evaluate. Details: surcharge calculator.
08 — Methodology & trust
- Same engine as /api/shipping-compare via
calcShippingCompare(emptyshipping_ratesfallback for UPS/FedEx modeled bases). - USPS GA:
lookupGaCommercialRate. Cubic: Notice 123GA_CUBIC_COMMERCIAL. - Methodology version:
2026-Q3.1-benchmark-1. Snapshot label: Version 2026-Q3.1 (12 Sept. 2026 snapshot). - Affiliate offers are not inputs to winner, runner-up, or spread.
- Confirm a label on your carrier account or shipping platform. DimMath is not a carrier.
Want to lower your shipping overhead? Use our Carrier Savings Engine to find the cheapest way to ship your products and optimize your bottom line.
Check My Savings →FAQ
Is September a new rate table?
No. It is a rerun against unchanged verified config.
Why do UPS and FedEx numbers look estimated?
When no shipping_rates row is present, DimMath uses the same modeled fallback as the Carrier Savings Engine. USPS GA and Cubic cells are published Notice 123 values.