In a small bowl, whisk together olive oil, minced garlic, lime juice, fajita seasoning, and salt to create the marinade.
Place the sliced bell peppers and onion in a bowl. Pour half of the marinade over them and toss to coat evenly.
In a separate bowl, combine the shrimp with the remaining marinade and gently stir to coat.
Heat a large skillet over medium heat and add the shrimp. Cook for 1-2 minutes per side or until shrimp are pink and cooked through. Remove shrimp from skillet and set aside.
In the same skillet, cook the vegetables for about 10 minutes over medium heat, stirring occasionally, until they are soft and slightly charred.
Return the shrimp to the skillet with the vegetables. Add chopped cilantro and squeeze additional lime juice over the top. Toss everything to combine.
Warm the flour tortillas on both sides in a dry skillet. Serve the shrimp and vegetable mixture with the tortillas, sliced avocado, and lime wedges.