I have to go with the same theme. Having been a builder for 20years and a home inspector...I see a lot.
When you get the supplies from the local distributor ..you get customer service, the correct parts that fit with the parts you ordered...and someone to yell at.
When you go the other route...you get a lot less responsibility on their end. Your builder may have to wait for the correct parts to get there when the ones you ordered don't work...same model #'s and all.
I have seen it firsthand , and the customer does not like when we pull out of the job for 2 weeks because we are waiting on their problem . There are always jobs which need to be started after yours, this creates a stress factor for both the builder and yourself ...one you don't need.
My opinion is from the other side of the fence.
You get what you pay for.
Good luck with your addition.