USPS: prices shipping not change after customer add more some products to cart

After customer add some products to cart, and they check grand total to checkout. They select state/province, fill a textbox their zip code and press Get a Quote button. After press Get a Quote, a list shipping methods display, have USPS. And they chang Qty product up or down, prices shipping of USPS not change, orther shipping have change. Who is know this problem. My site: http://cherryfones.com