I am always wary about "free" anything. There is an old business maxim which cautions, "There is no free lunch!" What that retailer really means is, "Shipping is included." Someone is paying the shipper, UPS, FedEx, etc. and it is the guy shipping the product. In order for him to pay the shipper he has to get the money from somewhere and it is in the price of the product. He may have a "deal" from the producer that gives him a discount on his product as a promotion. But he buys the product for X dollars and charges Y dollars so his gross margin is Y - X dollars. His net profit is his gross margin less expenses, taxes, etc. and one of those expenses is shipping.
I look only at the cost to get the product to my door and that is how I make a comparison. I should also add, I don't necessarily choose the lowest price product and I consider other things like service, friendliness, dependability, etc.