In case your store has items you fulfill somewhere else, you need to add delivering costs for those items independently on Shopify's end.
In situations where your client has both Supliful and non-Supliful items in their cart, Shopify will naturally take the most reduced accessible shipping rate from both item gatherings and join them together into one. So your client will see one delivery alternative available at the checkout.
In case you wish to make more advanced shipping rate combinations, there are some paid applications like Advanced Shipping Rules that will allow you to do that. There are other alternatives you may choose we are not specifically promoting to use this app, please do your own research on what fits you best.