traderjon, when placing small orders with CS, you must specifically ask for cheapest. Then they will send by mail at less cost. Their system is geared for UPS otherwise.
The USPS is making changes that are not user friendly. It seems that they are deliberately slowing down handling of Priority Mail. An important document I sent to my daughter in Colorado Springs, from here in Arkansas, in a flat rate envelope was actually held in the P.O. facility in CS for three days before they delivered it. When I called to complain I was told that Priority Mail is not guaranteed speedy delivery and for that one must buy Express or something else. And, do check out Parcel Post rates, they now appear to be higher than Priority Mail. Meaning that the P.O. has sneaked in a rate increase without actually calling it a rate increase.
I think we are pretty helpless with this situation. Only answer is to not run out of what you need and order well in advance. BTW, the PSI experience seems to me to be an anomaly, not the usual. I get good service from all the vendors I deal with, including PSI.