RAFIjzs I just add the expected e-packet price into the overall cost. yes if the items in the order come from 4 different suppliers then you will need to pay 4 different $2-3 USD e-packet shipping costs per item.
If the order is from the one supplier but is 4 items in the order then usually they only charge x 1 e-packet cost of $2-3 USD (depending on how heavy or bulky your items are too though, I sell quite small light items so it's easy for them to pack together and not have to pay more shipping - on bigger items I'd say they charge more for extra added just check with them if you sell large bulky items).
So you can either just work the expected estimated cost into the price of your item (by looking at a few shipping price costs for different items and suppliers to get a rough idea).
Or if you charge shipping to your customer then what you can do in shopify is set a weight value for each variant you have per product/item.
So say you sell a kids toys. And someone buys 5 toys from 5 different suppliers. You could have a shipping cost for one item at $9.95 USD and then any item added has to pay an extra $2 USD per item added (to cover the addition shipping from different suppliers).
The way you get shopify to do this, is you set a weight for every variant of your items and then go to shipping in shopify dashboard and set shipping classes also.
So weight 1KG per item variant = $9.95 (so if 1 toy is added to cart and equals 1 variant of that toy, say blue color toy, then the customer will only be charged $9.95)
but if the customer adds one more item to their cart and is from another product completely = 2KG weight in their cart now (because all your items variants have a weight class of 1KG, so 2 items = 1KG X 2, shopify recognises this and then increase the price + $2USD so the shipping is now $11.95 - which covers the extra shipping cost you will have to pay for an addition item)
To set up shopify so it understands this when customers checkout you have to go to shipping in the shopify dash and set shipping classes up,
So if you want to charge $2 USD extra per item added this is how you'd set it up in shopify (but you also MUST have all your variants for ALL your products edited so they are saved as a weight of 1KG so shopify knows when someone adds an extra product into cart that = 2KG and a new shipping class (higher rate) is generated at the checkout)
Set up shopify shipping classes something like
Shipping Class A = 1kg weight = $9.95 shipping
Shipping Class B = 1kg + 1Kg (2 items) = 2KG weight = $11.95 shipping
Shipping Class C = 1kg + 1Kg + 1kg (3 items) = 3KG weight = $13.95 shipping
and so on...
then go through all your product variants and set them to a weight of 1kg. Double check the checkout though it works because I had alot of trouble getting this to work but in the end it did work - use shopify support to help you, they should walk you through it (if you get someone who knows what they're doing which in my experience isn't always the case, some are really great and some don't seem to have any idea about the platform and I end up figuring it out myself).
It's probably easiest though to just add it into your overall product prices.. so just add an extra $2-4 USD (or whatever you like) on each item price, to help cover the cost of extra items added to the cart and then you don't need to worry about setting up shipping classes at all.