Setting the bill by either the overall cost or weight depends on the items that are being used in the sale.  Prestashop 1.5 allows you to set this option in the Administration Panel.  The following tutorial will walk you through setting the bill by total weight or by total price.

Determining the Billing method: Total Weight or Total Price

1. Login to the Administration Panel as an Administrator
2. Hover over the menu bar where it says SHIPPING, and then click on SHIPPING in the drop down menu.  You will see two existing sections, one called HANDLING and another called BILLING. Scroll down until you see the following entry:
   
   
   
   
3. Click on the radial button how to best determine the billing in your shop:  ACCORDING TO TO TOTAL PRICE or ACCORDING TO TOTAL WEIGHT
4. Select the green SAVE but in the top right hand corner of the screen in order to save your selection.

Depending on your products, you may want to select how you determine overall billing  - by total cost or by total weight.  Prestashop 1.5 gives you the option of using either for your shop. Click on one of the following to determine free shipping by weight or to determine free shipping by price.