Shipping for 2 itens or more (error)

[melissaposeyydon]

Hi,
I just set up my prestashop, and I went to test it and is miscalculating the shipping.
If the person buys an item everything is correct
If the person buys two or more items the shipping calculation is not based on the total weight of the order but associates a new shipping for each item.

For example:
Shipping cost from 100g to 500g is 3 $

Article A weighs 100g
The customer buys 4 items A

So 3 items x 100g = 400g falls within the range where the customer would pay only 4 $ (if it was correct)

However, what is happening adds up individually, so each item A is 4 $, forcing the customer to pay 3 times the 4 $ totaling 12 $ Shipping.

How do I solve ???