Some places charge actual shipping expenses while others will charge shipping and handling, which is to cover the expense of the shipping materials, like the envelop, and the cost of actually going to the post office to mail it.
Very often, (and I am not saying this shop is doing that) a small certificate is given and when you see how much the shipping is, you go on shopping to make it worth it. By giving you $10 they get you to spend much more than that. Is like coupons. If you don't use them smart you will end up buying items or brands you normally would not use. I argue with DH about this because I find brands that are as good and cheaper without a coupon than the ones he wants to get just because there is a coupon.