This just happened to me twice. I found something close to what I was selling, clicked on "sell something similar" as a shortcut, then modified the listing to what I was actually selling. My item(s) was quite a bit larger and heavier than the original item. When I changed the shipping dimensions and weights, the costs for all shipping options updated EXCEPT for USPS. Those rates stayed the same whether I put in 1 pound or 100 pounds. I assume they were the rates for the original item. When I created a listing from scratch for the exact same item, the rates were correct. If I hadn't noticed this, I could have been screwed big time for shipping.
