I use USPS more than I use UPS, I find them cheaper, and actually easier, I also don't find them complicated at all(I use the online shipping calculator to get an idea the ship the item at a local post office). For example, I shipped a Farmall C grill to Bloomsburg, PA last year, weighed it on a scale I have here, checked online for a price and also checked UPS who wanted something like $50 to ship it, USPS shipped it for $20. I don't use the flat rate boxes as I find it's cheaper to use my own box and weigh the package and send it at my local post office.
I find both company's to be extremely easy to use...but I have a UPS terminal across the road from where I work, and they have a shipping office. I find both of their online calculators to be darn near spot-on! UPS is always within $1-$2 of the estimator when I go ahead and ship. USPS is never an issue because I pull the quote, then print the label right then, and have it picked up from home. I have never had a problem, and simply use a bathroom scale, and round up to the nearest full pound.
I have NEVER found the same box to ship for more then $15 difference between the two. A $30 spread doesn't even seem legit.
Today I was surprised...I shipped a 40" belly grader, 51lbs, and UPS beat USPS by $11. I shipped UPS. Likewise, I shipped two, 25# weights in a large USPS flatrate box for $15 yesterday. Without flat rate, that same shipment is $43, USPS. Flat rate rocks for tractor parts and weights, but of course, the prices of the weights have gone up since shipping them became reasonable.
I have found more packaging/handling issues with USPS than UPS, and I won't even allow someone to ship something to me using FedEx. We don't have time for all those horror stories!
One big bonus for USPS...Saturday is a delivery day. But will that last?