I sell a set of 6 hand grippers for $60 free shipping to USA, weight 3lbs 12 ounces
Say a customers adds that to the cart.
I sell another product for $39, $5.99 shipping to the USA, weight 1lb 1 ounce.
Now they buy these two products, total of $99, weight 4lbs, 13 ounces, but shipping needs to only be $5.99.
To summarize, my heavier item needs free shipping and the lighter items cannot have free shipping. For the $39 product if I add shipping charges to the selling price, now making it $44.99 with free shipping, my price looks higher than my competitors.