I'm had the same problem. I assumed that I had too much weight but now I wonder. I'm trying to ship first class parcel which should work for anything under 13 oz. Looking at the USPS rate calculator guide it looks like their should be "letter flat parcel" type under firstclassmailtype, but maybe that's the under 3/4" rule. I don't know. I hate to start digging through all of WA code but maybe it's down to that. :-(