1) Chop shrimp then add to a mixing bowl with lemon and lime juices. Allow to marinate for 20-30 minutes.
2) Stir in cucumber, red onion, tomato, cilantro, avocado. Season with salt and fresh ground pepper. Refrigerate until chilled.
3) Bake pre-made corn tostada shells in the oven for 3-4 minutes at 350° F, or microwave for 30-45 seconds.
4) Using a warm tostada shell, top 2-3 tablespoons of chilled ceviche onto the shell, then garnish if desired. (Tip: Use a slotted spoon to drain the ceviche before placing it on the tostada).
Alternatively, serve ceviche in small, individual bowls with chips on the side.