I wanted to add my 2 cents as a contractor.
I want to start by stating that I am only replying to general practices that I adhere to, I do not know what you have worked out with your guy. I also want to point out that just because someone is working on your flip does not make them a contractor.
I charge in whatever way that makes my client comfortable. Simply put, if you are not paying a premium, why would I supply consumables. If you are paying a premium, I would expect to supply the consumables. I have a guy doing a flip for me in another city, his prices seem a little low to me. I supply him with all consumables. As to collecting them at the end of the job, feel free, but as one of the people here stated, act cheap...get treated like you are cheap. Most consumables are just that, consumable and if used for your job is most likely somewhere into the life of the consumable and may not be worth you collecting.
It is important for all investors to understand that we do this to make money, not to make money for you at the detriment to us. You are in investing and should reap rewards for this, but I am not responsible for whether or not you make money. Find someone that will work with you and if you treat them right, they will be around and help you make money from your investment. One way to achieve this is to offer some type of small pay and some percentage based on your profit, that would get buy in from the person doing your work.
I hope this helps.
Rick