It might have been stated a while ago, but I guess I'll make my point:
If you look at the price of a fish at a pet store and then online at a popular site, yes, the site would be cheaper, until you get to the shipping. You can pay $3 more in store or you can get it cheaper with a $30 shipping cost online.
I like to go to the store especially if it is one of those mom-and-pop shops. There is one by me and it has to be the best pet store I have gone to. I would gladly pay a few extra bucks to support it and keep it running. Their fish are very well priced and very healthy. Each tank is set up for the type of fish in it, to minimalize the stress of the fish when in a temporary home. The dwarf gourami tank has tons of plants, the mbuna tank has lots of rocks and no plants, etc. Some of the prices are actually cheaper than online!
The bottom line is, it is up to you to make the decision. It is your responsibility to be an informed buyer, and if you don't you will quite literally pay the price.