I was listing an item and I put 3 pounds in the weight. Dimensions 15x12x2. The example that it defaults to is shipping cost to Chicago and the shipping cost said 11.41. I then weighed it again after packing it and realized it was only 2 pounds. I literally JUST changed the weight from 3 to 2 and the new weight became 12.60. More than the higher weight. WTH?
You can always check at USPS.com and see what it shows.