Peel, de-vein, and pat shrimp dry. Season with the fajita seasoning and 1 tbsp of oil.
Preheat Cast Iron skillet and place remaining oil, Bell Peppers, Onions, and remaining seasoning. Stir well and cook for about 7 minutes till onions and peppers become soft.
Remove onions and peppers from skillet and place seasoned shrimp on skillet. Cook for about 2 minutes or until shrimp is a light pink. Flip shrimp over and cook for another 2 minutes or until shrimp is no longer opaque. Do not over cook the shrimp, it will become rubbery.
Heat flour tortillas on a clean skillet until warm, and keep in foil or wrapped in a clean kitchen towel.
Serve shrimp fajitas in tortillas and top with guacamole.