Yes, your boss can do this, unless you had an actual employment contract which specified your delivery-related duties. Employers set the duties and job responsibilities of their staff; they can change those duties at will, as long as there is no contract to the contrary. They also do not need to pay employees extra for doing more work; the only obligations regard pay are 1) always pay at least minimum wage; 2) pay hourly staff for all hours worked; 3) pay overtime when nonexempt staff (which includes all hourly staff) work more than 40 hours in a week.