Amazon is absolutely great for things like this (no sales tax & no shipping) BUT, like most retailers in the Apple supply chain, they will not have inventory for several weeks. If you can wait to get a new computer for a month or so, then they become a very viable option. If you want your computer immediately, or if you have some kind of discount (student, education, etc.) then the prices are pretty much a wash and you can get the computer sooner.
It's sometimes a bit risky to buy from a middle man vs. direct from Apple, but Amazon is one of the better third party options.