Zone is the variable most sellers price blind, because a single test shipment only shows one point on the grid. USPS and UPS both price by distance band between the origin and destination ZIP codes, so the same service can be inexpensive to a nearby zone and considerably more expensive to a farther one at the identical weight. Reading a rate table means reading the whole grid, not the lowest number printed on it.
Dimensional weight changes the weight tier a package lands in before the zone is ever applied. UPS and FedEx bill the greater of actual weight and a volume-based weight, calculated from the box's length, width, and height, on the package services the rule applies to, which covers the ground and express services most sellers use. USPS applies that same calculation only to qualifying Priority Mail Express, Priority Mail, Ground Advantage, and Parcel Select packages over one cubic foot addressed to Zones 1 through 9, so a light item in a snug box can stay on actual weight where the identical item in a larger box cannot. Checking the box's dimensions against that one-cubic-foot line is worth doing before assuming the table's lowest weight-based row applies.
Commercial pricing versus retail-counter pricing is the other gap worth knowing before you quote a customer. USPS publishes a retail price for buying a label at the counter and a lower commercial price for buying it through shipping software, with no volume minimum, and UPS likewise separates its published daily rates from the discounted rates shipping platforms pass along. FedEx is different: the commercial column on its two tables is account pricing negotiated with FedEx directly, so it varies by shipper and is not a tier you reach just by using software. The tables below give both columns where the data exists, because pricing shipping off the retail number quotes against a price nobody with a shipping account actually pays.
USPS, UPS, and FedEx do not raise prices on the same schedule or by the same amount, so a rate table is only current as of the date printed on it. Each guide below states its effective date; re-check a table older than the most recent increase before relying on it for a customer-facing shipping charge.
Use the table for the service you already know you want; use the comparison guides instead if the question is which carrier or which service to pick in the first place. I'd Ship That prints every USPS and UPS service below at discounted rates, priced below USPS commercial, with the full price shown before you buy. The FedEx pages below are reference tables kept because sellers compare against them; FedEx is not sold through I'd Ship That.