This holiday season a vast number of people bought things online on sites touting "free shipping". That's all well and good, if it truly was "free".
My experience is that the Free Shipping moniker is often used as an attraction to customers who then actually spend more to get the same items as they would on the site that charges shipping. Face it, the costs to the seller are the same to ship it to you, whether or not they charge the buyer.
So either the seller accepts a smaller profit margin, the seller is getting its goods for less due to volume (like Amazon and Walmart versus mom & pop), or the seller is actually charging more for the original item or the accessories that you "have to have", etc.
I will sometimes pay for shipping because the total price is actually less. Or I will pay for shipping because I want to support a small business in preference to a big seller.
The bottom line is be careful and pay attention. Caveat Emptor.