I think some places,Amazon comes to mind, pressure the shipper into very low cost shipping. Once in a while I buy brewing stuff from them because the total price is way lower.
I will not buy from sellers that low ball the price only to gouge you on shipping. Flat rate like from our host is nice but kind of makes you have to get lots of stuff and it does suck if the one thing you want if it's not included. Pays to shop around and not too hard online.