I have a 115lb shipment of small metal parts that needs to be across the country before Wed. Both FedEx and UPS quoted me ~$750 to ship it in a 12"x8"x8" OSB box via 2-day shipping. Fedex one rate boxes have a weight limit of 50lbs. The small boxes are $31/ea to ship. I just finished breaking the shipment up into 3 parts. Heck, next day would be ~$300.
There's a reason for it. Package sizes and weight limits are optimized for workers who are going to carry them. So yes in your case the total weight loaded on to the truck or plane may be the same, but loading/unloading 3x40 lb boxes and carrying them to your front porch is very different from doing the same with a single 115 lb box.
Airlines work the same way. 2x50 lb bags cost $30 each. One single 100lb bag has a $200 surcharge.
Usually large items go through an entirely different delivery network because the whole parcel delivery chain will be designed around weight limits - I.E. that weight might not be able to go through the automated sorter, and there is very limited capacity for manual workarounds for individual parcels.
Let me recommend pirateship.com for discounted USPS/UPS rates. Not affiliated, just a happy customer.
It gives me a UPS 2nd day air quote of $338.90 from 27518 to 95050 for a 12" x 8" x 8" box weighing 115 lbs. That's supposedly 66% off retail ($1000.12).
But as sibling comment mentions, think of the person delivering a 115 lb package. I semi-recently shipped some treadmill parts to a recycler and they went via three boxes (the recycler paid for them).
Negotiated FedEx and UPS rates are often 90% off the retail price or better. If you can get the box sent from a shipper that has a daily pickup from UPS and reimburse them, you may save a great deal of money.
