Problem after update ,shipping cost error 2 product in the cart

hi all , I’m new to cs-cart



My shipping cost are calculated by weight



if I enter 2 product in cart, one product insert after update + one product insert before update . shipping cost result wrong



if I enter 2 product in cart ,insert before update:shipping cost result ok

if I enter 2 product in cart ,insert after update:shipping cost result ok

if I enter 2 mix product in cart ,insert before and after update:shipping cost result error



products have the same weight