USPS is sometimes cheap to ship certain packages but for most packages the UPS is the cheapest option for shipping especially heavier packages.
I once shipped something through USPS that weighed 25 lbs and it was a small item and it cost $50.00 to ship it and it was something I sold on ebay.
The buyer paid the shipping so it wasn't a problem and they wanted it shipped through USPS so that's who I shipped it with.
But if I were to ship that item through UPS the price would have been around $20.00 to $25.00 or sometimes cheaper depending on where it needed to be shipped.
UPS tends to handle packages safer as well and better so I usually ship through UPS the items I sell on ebay and find them much cheaper unless someone wants me to ship USPS and will pay for it.