Speaking only for myself on this, I don't have time to order parts on line. Just pull into the supply house parking lot, enter the store, purchase needed parts, and leave. What happens when that part you ordered on line, is faulty or not the part you ordered? Tell the customer it'll be a few more days without, because you have to ship the parts back and fourth. I can't see any advantage to buying anything on line.
I wouldn't ever buy parts online. Why would I? I drive 15 minutes down the road and get what I need. Who wants to wait a few days for something I can have so much faster? I will assume you are a homeowner fishing for places to buy parts from.
That means you don't have a license and are trying to find out where to get what you "think" you need, or maybe you're just trying to get prices to see if you think you got ripped off on your last service call. Right????