Carriers price parcels by the space they take up as well as by what they weigh. They multiply the package's dimensions and divide by a published divisor, turning volume into a billable weight. If that number beats the scale weight, you pay for the larger one. A light hoodie in a big box can cost far more to ship than its weight suggests.
Apparel is light and squashable, so packaging choice decides most of it. Soft poly mailers rarely trigger dimensional weight, while rigid boxes for a branded unboxing can push a parcel into a higher bracket. Divisors and thresholds differ by carrier and service and change over time, so check the current figures on each carrier's site.
When setting shipping rates, measure real packed parcels, not empty boxes. See shipping zones explained and packaging and unboxing.