Make the fajita seasoning. In a small bowl, stir together the chili powder, cumin, garlic powder, paprika, oregano, salt, and pepper.
Season the shrimp. In a large bowl, add the shrimp, two tablespoons of olive oil, the juice from half a lime, and the fajita seasoning. Toss it all together until the shrimp is well coated, then set it aside to marinate while you cook the onions.
Sauté the onion and bell peppers. Heat the remaining two tablespoons of olive oil in a large skillet over medium heat. Add the onion and bell peppers, and sauté for 4 to 5 minutes, until softened and the onions have started to caramelize. Transfer to a plate.
Sauté the shrimp. In the same skillet, add the shrimp and cook for 2 to 3 minutes on each side, until they turn pink and opaque.
Combine the shrimp and fajita veggies. Add the bell peppers and onions back to the skillet, give everything a toss, and squeeze more lime juice on top. Serve immediately with tortillas and extra toppings such as sour cream, pico de gallo and guacamole.