that do this very same thing. This should have been negociated during the contract phase of the job.
Now, consider this, as I do, if I have to go and do the shopping and wait in the lines and haul the stuff to the site. I do also charge a shopping fee. You have to think of this, my time is money and if I am doing stuff for you like shopping and buying stuff, I need compensation for this. When I go and get stuff at a home center for a job, I am on the hourly clock and I tell the people up front of this. I tell them that they can get it all and have it on the site or I will do it for them. I charge for my time.
Now 13% is not unreasonable for his time and expense, and gas and all that stuff.
I see your side also, but also consider his/her side too. I would bite the bullet on this and the next time you need supplies you do it and save the $$$$.
Maintain your relationship with the contractor if not times become really hectic.