Season the shrimp with the taco seasoning. Heat the olive oil in a large pan, and cook the shrimp about 2-3 minutes per side over high heat, or until fully cooked. Set aside.
Mash the avocado in a small bowl and mix with the lime juice.
To arrange the tostadas, spread 1/4 of the avocado onto each tostada, and season with a pinch of salt & pepper. Top each tostada with 3-4 shrimp, halved tomatoes, diced bell peppers, diced scallions, and feta cheese. Squeeze extra lime juice over the tostadas, if desired. Serve immediately