Hi Nicolas,
when I tested buying two different items, the calculation of the delivery cost in the checkout is slightly different, please see the attached screen shots. Delivery for one item = $ 15, two items = $29.99
I am using "shipping price per product", your solution last time fixed the calculation for buying several quantity of the same item.
Try to change the line:
'price' => (int)$rate->shipping_params->shipping_price_per_product,
to:
'price' => (float)$rate->shipping_params->shipping_price_per_product,
in the file plugins/hikashopshipping/manual/manual.php
Is there other thing that we need to change within the codes?