I would like to have a fixed price for each chart.

Hi,

I'm not sure what you mean.
If you want the shipping fee to be global to all the products in the cart, you can enter it in the "price" field of your shipping method. That way, regardless of the amount of products, the shipping cost will be the same.
If you want to set the shipping cost per product so that the total shipping cost on the checkout is the sum of the shipping cost for each product, you want to activate the "shipping price per product" setting of your shipping method, and instead of adding the cost in the "price" field, add it in the "price per product" field which will appear after you activate the "shipping price per product" setting.

Hi,

Could you provide screenshots of the settings of your shipping methods ?

Also, I'm not able the reproduce the problem on your website. Would you have some precise instructions to be able to reproduce it ?

Hi,

The shipping methods are fine.

Could it be that you configured different warehouses in the System>Warehouse menu and assigned different products to different warehouses ? In that case, you would have one shipping method selection for each warehouse on the checkout, and the total shipping cost would be the sum of the "price" of the shipping method selected for each warehouse.

Hi,

You need to run a mass action via the menu System>Mass actions. Just use one action "update the values" on the column "product_warehouse_id" with the mode "int" and the value 0 and click on the "process" button and it will do it.

Hi,

Precisely, yes. In that case, the products with a warehouse and the products without a warehouse would be separated in the checkout, each group with its own list of shipping methods and that would explain the fact you had two the shipping cost.