I have found the opposite to be true. From where I live, there really is no difference between Priority 2 or 3 day and Ground Select or whatever it is. Priority always seems to take the same amount of time as Ground Select and they won't refund anything if the package is a day or more late. Pirateship helps out a lot. A recent kit sale i made showed over 20 bucks for the cheapest USPS Gound rate as opposed to 11bucks for UPS Ground. It does vary and sometimes UPS is more. Getting quotes on different days seems to make a difference also. As far as the duct tape goes, I've experienced the same as you. I got packages taped that way with no problem but local office wouldn't let me ship the same parts in the same container back to where they came from. I think some USPS people think there is metal in it or something, like the high temp aluminum duct tape does. I don't even attempt that any more.
Type at you later,
Dan McEntee