Is it permissible in Sharia for me to add $500 to the car's price for my effort and delivery, knowing that the original seller is aware of this?

Commission or brokerage fees may be taken from both the seller and the buyer, or from either of them, provided that the person from whom it is taken is aware of it. If the increase ($500) will be paid by the buyer who alone requested the car, then it is not permissible to take it except with his knowledge.