Preheat oven to 425 degrees. Using a measuring cup(1 cup) add 1 tablespoons of white vinegar and enough milk to make 1 cup. Set aside. In a medium sized bowl add the flour,sugar,salt,baking powder, baking soda, butter, shortening and mayonnaise. Using either a fork or pastry cutter blend until crumbly. Create a well in the center and add milk. Stir until incorporated. Dough will be sticky. Dust your work surface with flour and add dough. Sprinkle the top of the dough with flour. taking a scraper from the sides fold over the dough five to six times adding only enough flour to make the dough workable without being sticky. Pat dough into a circle one inch thick. Using a two inch round cutter cut out 12 biscuits. Place the biscuits side by side on a baking sheet and bake for 15 to 20 minutes or until tops are golden brown. Remove biscuits and brush the tops with melted butter.