USPS Priority Mail is the happy best-of-both-worlds blend of lower cost and fast speed. It has three kinds of rates: flat, by cubic dimension, or by weight.
Automatically toggles between Weight-Based and Cubic rates for Ground Advantage to provide the most economical rate for ground shipping.
For those parcels too large for Cubic rates, this offers substantial savings, though be mindful of large boxes due to Dimensional Weight pricing.
USPS Priority Mail Cubic can save you money on smaller parcels, even small-but-heavy ones. As long as your parcel is close to shoebox size (or smaller) you can ship 20 pounds or less with no additional fees.
USPS's quickest offering. Overnight delivery in city regions, with a two-day span for more remote areas.