You have to be firm and insist on paying for the materials yourself. There are many reasons the contractor may pull out to get the money from you: you’ll have to drive yourself over there; my brother (cousin, in-law, compadre) gives me a great deal; I have to see what they have available, and many more. But one way or the other avoid giving the contractor money to buy materials.

You often can pay the driver upon delivery. I used to call the depot in the morning, after getting a list from the contractor, and place my order. They would bring the receipt and I would be able to cross check the delivery.

Many builders are honest, but bad with money. As Bob said, some have “liquidity” problems. And some are dishonest.