I mostly use Amazon Prime for dry goods. Big ticket stuff like rodi I get straight from the manufacturer cause it makes costumer service easier if I need help setting it up. There are lots of options for livestock, and they have sales once in a while that make it worth following the vendors on Facebook.
I think the best deals for free shipping on stock are pacific east aquaculture, saltwaterfish.com, and bluezooaquatics. It's a trade off when you buy cheap fish and corals, there are cheaper vendors but they have terrible reputations for the health of the stock. Liveaquaria and their divers den are the gold standard for online ordering, but they have high minimums.