Three variables decide almost every carrier flip on this site: the package's weight, the zone it is traveling to, and whether the box counts as dimensional. Weight alone rarely settles it, because a service that is cheaper at one weight can lose at the next weight break, and a service that wins to a nearby zone can lose to a coast-to-coast one. Reading a single headline price and assuming it holds for every shipment is the mistake these comparisons exist to correct.
Dimensional weight is the variable that surprises the most sellers, because it is billed on the box, not the contents. 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 volume-based 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, which means a light item in a right-sized small box can dodge the charge on USPS while the identical item in an oversized box cannot. The box, not the carrier relationship, is usually the first thing worth checking.
Zone is the other half of the answer, and it is easy to under-price because a seller usually tests a rate to one destination and assumes it holds everywhere. The same package can be the cheaper USPS option to a nearby zone and the cheaper UPS option to a distant one, because the two carriers' zone-based pricing curves do not rise at the same rate. Checking the zone for your highest-volume destinations, rather than a single test shipment, is what keeps a routing rule accurate as your customer base spreads out.
I'd Ship That prints discounted USPS and UPS labels. FedEx appears in several of these comparisons because sellers compare against it, and those pages are reference material: FedEx is not sold through I'd Ship That.
Each comparison below names the two options being weighed and states where the flip happens, so the page can be used as a lookup rather than read end to end. If your package matches the weight, zone, or box size described, the flip point is the number that decides your label. I'd Ship That rate-shops USPS and UPS on every order and, with Ship Intelligence (Pro), applies the cheaper valid option automatically once you would rather stop checking by hand.
Pick the comparison that matches the two options you are actually choosing between. If you have already chosen a carrier and only need its price table, go to the rate guides instead.