Hi Mohamed,
I think mine might be a little trickier then that, unfortunately.
What I've got is several products with different variants of weight, for example a product might have a option to be either 30kg or 20kg. So I've created a chararistc called Size and created these 2 variants.
Now I've set each variant from the product page to be the correct weight and created 2 different shipping rules, for example let's say £10 for 30kg and £5 for 20kg. However, since the 20kg product still falls into the range of the 30kg rule, they are getting charged £10 instead of £5.
Hopefully I've explained this correctly, it's getting quite confusing!