I tend to agree. Foster a relationship with your local supplier. I don't mean the big box stores. Go with the supplier who is local and employs your neighbors.
One possible drawback ordering online is your builder has a relationship with your local supplier. He builds his schedule around the delivery of your stuff.
If you order online and there is a delay, you stand to lose the attention of your builder. He'll move on to someone else's project to keep his guys busy.
Every time he has to mobilize and de-mobilize it costs him money. If it starts costing him money, you might see some added expense passed along to you -and quite fairly.
You can try to negotiate with your local supplier for all your fixtures (as a package). All told, it turns into a 'big ticket' item and the rules are the same as when buying a car, for example.
Your local supplier has local competition, too (including the bigbox stores). He'll need to have a sharp pencil when he figures his bottom line. He knows his competition and I am sure his pencil will be sharp. No harm in 'offer' / 'counter-offer.' That is the age old dance.