As Mike said, Pay n Spray works on the gta sa storymode concept so it cant be changed. Maybe if possible, new vehicle repair stations can be opened at the different garages of the story mode houses. As soon as u enter the garage and reach a checkpoint, a menu would appear which would show up with different options like engine repair, tire repair and so on, just like the mechanic menu, only the charges would be much lesser like $200 engine repair and $50 each tire repair. There would be much more repair stations and would cost even less. TIP : name that Uncle Mikes Workshop :p