PrestaShop Shipping Cost Calculation Fix

I have a PrestaShop site version 1.7.8.7 and I have configured various shipping methods specific to each item based on their individual shipping costs. However, I am encountering an issue: when customers purchase multiple items that require different couriers, even though it's the same customer and the same cart, the system erroneously creates two separate orders with the same order reference, instead of one. I need to modify the system so that it always creates a single order with a unique shipping price. It should select and apply only the highest courier cost and then add only the additional shipping costs for each selected item.

Key Requirements:
- Fix the shipping cost calculation to ensure only the highest courier cost is applied.
- Implement a system to add variable costs based on item attributes.
- Ensure the solution integrates smoothly with the current PrestaShop setup.

Ideal Skills:
- Extensive experience with PrestaShop 1.7.8.7
- Strong understanding of ecommerce shipping cost calculations
- Prior experience with modifying PrestaShop shipping modules
- Ability to provide a clean, efficient solution.