In a medium bowl, combine the shrimp and lime juice. Ensure the shrimp are fully submerged in the juice. Cover and refrigerate for 30 minutes to 1 hour, until the shrimp turn pink and opaque.
Once the shrimp are ready, drain off some of the lime juice, leaving about 1/4 cup in the bowl.
Add the red onion, cucumber, tomato, jalapeño, cilantro, salt, and black pepper to the shrimp. Mix gently to combine all ingredients.
To serve, spoon the ceviche mixture onto each tostada shell. Top with avocado slices and serve with lime wedges on the side.