In a medium bowl, combine the shrimp and lime juice. Ensure the shrimp are fully submerged in the juice. Cover and refrigerate for about 30 minutes, or until the shrimp turn pink and opaque.
After the shrimp are ready, drain excess lime juice and transfer them to a large mixing bowl.
Add the red onion, cucumber, tomatoes, jalapeño, cilantro, salt, and black pepper to the shrimp. Gently toss until all ingredients are well combined.
To serve, spoon the ceviche mixture onto each tostada shell. Top with avocado slices and serve with lime wedges on the side.