USPS, UPS, and FedEx all follow the same general sizing guidelines for standard shipping costs. Measure the length (the longest side) and add it to the girth (measuring continuously all the way around). You’ll want this to be less than 108″ for it to be a non-oversize box. Measure carefully, they will seldom overlook more than an inch or two.
A typical frame box is 40″ x 24″ x either 6″ or 8″. That calculates out to just 108″. If it’s 108″ it can ship Priority USPS, if it’s bigger then it needs to go Parcel Post Oversize and it might cost 50% more plus it takes longer. Now you guys who ride a frame bigger than 60cm or so have a little problem. Your frame needs a bigger box anyway, so your shipping costs either way will be a bit more. Fact of life.