I am going to give some details so we can save both parties time.
I have shop that it's using prestashop 1.6.x.
The shop it's using Advance Stock Management and has 2 Warehouses. There are no products with manual set quantity.
Both warehouses are available for each product.
We have Product A and Product B.
Product A has 3 products in Warehouse 1.
Product B has 3 products in Warehouse 2.
The handling fee for shipping it's $20.
If I am adding 1 Product A and 1 Product B to my cart I am getting the shipping fee doubled to $40.
I require a modifications that will allow multiple products be bought between carts with a single time fee, no multiple or triple.
This is not a single time job, the shop will require regular updates and I will need further the fixes to work in future versions that will be paid, so bid according.
If you tread this with seriousness there will be future works.
As you can see this it urgent, if you have no experience working with prestashop don't waste my time and yours.