Shipping form multiple vendors

[raucousrags]

I'm starting an apparel store and will use more than one vendor.  Each vendor has different pricing on shipping as well as Shopify having shipping rates. I would like to offer free shipping but need to be able to average the shipping costs.  My confusion begins with Vendor + Shopify shipping rates which appear as 2 different figures. Certainly I won't pay for shipping from vendor & shopify.  

Does anyone use different vendors and offer free shipping?  How did you do arrive at your shipping costs?

Thanks For your Help