Select the most expensive shipping method when several products are in the cart

Hi,

You can't do this with warehouses.
The goal of warehouses is to provide different shipping costs for different groups of products and sum the shipping costs of each group into one total shipping costs.
So first you need to undo what you did with warehouses: unselect the warehouse selection in your products and shipping method and then delete the warehouses.

There is actually no mechanism to work the way you want with the manual shipping plugin included with HikaShop at the moment.
So this would require the creation of a custom field of the table "product" via the menu Display>Custom fields, so that you could enter the shipping price of each product when you edit them in the backend. And then, a custom shipping plugin would need to be developed to take into account the value entered there for each product in the cart and use the higher one as the shipping fee.

Actually, that capability is quite easy to add on our end so we'll work to add it for the next version of HikaShop (in the 5.0.2 to be released in January).
If you want to use it right now, what you can do is to change the line:

$price += $rate_prices['price'] * $row->cart_product_quantity;

if($price < $rate_prices['price'])
	$price = $rate_prices['price'];