Shipping based on the weight doesn`t count the cost correctly.

For instance, we can take the product from the link and add it to the cart.
This wallet weights 0,5kg.
Cyprus as the delivery country activated.
Calculation Method is SUM
the table: Cost per order Sum(just for explanation)
weight 0 1 17,4
weight 1 2 5,9 23,3
weight 2 3 5,9 29,1
weight 3 4 5,9 35,0
weight 4 5 3,5 38,5
When I have 1 wallet in the cart it shows 17,4 that`s correct
Then when I add 1 wallet more to the cart that is 1kg, it shows 23,3 that is correct as well.
But after I add 3rd wallet inside the card, started the problems:
Instead, to show 23,3 (because the weight is between the range of 1-2kg)it shows 5,9.

I add one more to the cart =4 = range from 2-3 it shows 11,8 instead of 29,1.
Next 5= 2,5kg range from 2-3, it shows once again 5,9. that is incorrect must be 29,1.
And so on……

The page I need help with: [log in to see the link]