Summit is great. You pay fixed price up front and its delivered to your door. You can get carried away very fast though. As its like xmas all over each time. From memory you get it within 7 days as well.
Heads, cams, valve springs, intakes, throttle bodies etc all availble
I bought a pair of Trickflow cnc heads from Summit year before last and was a pretty painless purchase, taxes paid and like you say easy to do. I just don't find the website the easiest, they send catalogues every few months which is pretty cool.