It depends on whether you required them to drop off these plans. If the plans could have been delivered on time by some other method and you did not actually tell them to drive themselves then you could argue that you do not have to pay. But bear in mind that a courier would have cost you money anyway. If you told them to do it then you are obliged to pay, it is not reasonable to expect employees to follow your orders at their own expense.
Did he deliver a can of wd-40 to you with the expense report? Yes!!! You need to pay up. Next time go deliver it yourself, and don't say it was on his way home. Maybe he was not going home after work. You should be thankful to him for going the extra mile. You should be asking your self why you are even thinking about this. Shame on you!
Are salaried employees entitled to mileage reimbursement?
It depends on the terms and agreement of the employment. If you have promised them the mileage then you have to reimburse otherwise you need not to.