My rule of thumb is that every not-heavy product has about $7 of shipping costs baked in somewhere. For cheap items, there isn't enough markup to cover those costs, so they are usually higher priced than at retail. If it's $50 or more, there usually is, and the amazon price will be competitive or better than retail.

For heavy things the shipping ding is bigger, but they also usually cost at least $30. No one bothers to sell $5 items like 50lb bags of basic sand on amazon.