I want to make it so that shipping costs will be automated where the customer is receives our negotiated rate with with our carriers + a 15% upcharge.
So far, all I can figure to do is create shipping profiles based on weight of the garment, and each product has to be manually assigned to a profile (time consuming when adding in whole new collections). However this isn't exactly working ... for instance we are shipping out sometimes 100 face masks at a time, but each face mask is less than a pound and the assigned "box" is an envelope, when really we'd ship in at least two large boxes. How can we initiate a more streamlined and effective process?
The required shipping rule is quite complex and since you already tried this with Shopify, you know this is not possible with it.
If you are open to using an app for this purpose, then Shopify Multi Carrier Shipping Label App is the best option. It allows you to set multiple shipping rules including your specific shipping requirements. For any queries feel free to reach out to the support team.