Custom code within opencart

General information for the website: we sell electronics online
Kind of development: Customization of existing website
Description of requirements/features: i have two shipping methods setup within opencart:
1. Product based Shipping (priced per product)
2. Royal Mail (Weight Based)

i have two sets of products from two different suppliers in my store. one provides a shipping cost per product and one doesn't.

supplier 1 will provide the shipping cost, so i have this applied to the product using "product based shipping" method.
supplier 2 will provide the weight, so i use "royal mail" method to apply shipping cost.

problem is if the customer adds multiple products to their cart from different suppliers it won't combine the shipping charges it will offer them a choice of one or the other.

i need it to add shipping costs from both methods to the cart if products from both suppliers are added
Payment methods: Paypal
Extra notes: