If yoiu read the fine print in the ads, it says postage and handling covers actual shipping costs, plus "handling" is a term so broad they use it to included operating costs, overhead, and manufacturing costs.
So basicly, the sunglasses you're buying cost less than what you're paying to make and they're still making a profit. Besides that, they probably help pad profits by selling your name to junk mail lists.
Save yourself the hassle and buy cheap sunglasses locally.