Whenever shopping online, auto parts included, you always have to consider the total price - including add-ons such as tax and shipping. Rarely have I found Rock Auto to be the best deal. Sometimes the bottom price is comparable, but other places offer service that is so much better that I will go with the other options.
For example, I just ordered spark plugs for my SL550. I shopped Rock Auto, Amazon Prime, and FCP Euro. Amazon Prime and Rock Auto both wanted $5.99 each, so $47.92 for 8. Rock Auto wants $4.99 for their cheapest shipping, so now $52.91 for Rock Auto. FCP Euro wanted $6.38. Total is $51.04 so I qualify for free shipping. Free shipping at FCP Euro is faster than the $4.99 shipping at Rock Auto. Plus FCP Euro gives lifetime replacement on every part, including wear items like spark plugs. So I paid $3 more than I could have gotten them from Amazon, but $2 less than Rock Auto. And I will never pay for spark plugs for my SL550 again.
I will admit that I like their parts search engine, so I will use it quite often, and of course compare prices while I am there. But I always calculate shipping before deciding.