Preheat oven to 375°F (190°C).
Cook pasta shells according to package directions. Drain and set aside.
In a large skillet, cook sausage over medium heat until browned. Drain excess fat.
In a mixing bowl, combine ricotta, 1 cup mozzarella, ½ cup Parmesan, egg, Italian seasoning, garlic powder, salt, and pepper. Stir in cooked sausage.
Spread 1 cup marinara sauce in the bottom of a baking dish.
Stuff each shell with the sausage mixture and place in the baking dish.
Pour remaining marinara sauce over the shells. Sprinkle with remaining mozzarella and Parmesan.
Cover with foil and bake for 30 minutes. Remove foil and bake for an additional 10 minutes, or until cheese is bubbly and golden.
Let stand for 5 minutes before serving.