It's a really good point that you raise, and I must admit that I have always put it down to volume shipping discounts allowing them to offset the shipping price in the item price, but who knows if that's accurate.
Still, at 5-10 dollars with free shipping, why not try it. I know I ordered a bamboo kitchen item for home on one of those deals. Cost me under 2 bucks to buy the product and they threw in free shipping. I had no issues at all, product arrived quickly and the quality was great, so however they do it, it's a good thing : )