Over the past year I've made a dozen or more Rock Auto (RA) purchases, several hundred $ worth. One thing I've found is you really should take the RA part number and compare with Amazon Prime, quite often the free shipping with Amazon Prime means you save many $$.
Now if RA ever institutes free shipping, then it's no contest.
Hummm, RA, if you're monitoring this thread, what about a membership option that would offer a percentage discount and free shipping.