Chop pre-cooked salad shrimp fine and serve as ceviche salsa with tortilla chips.
Place shrimp, lime juice and orange juice in a large resealable zip bag and marinate in the refrigerator for 2 hours.
In a large bowl, combine remaining ingredients except for sliced avocado, chopped cilantro, and tostadas. Stir to combine.
Add shrimp and half of the marinating liquid and toss with cilantro. Serve on tostadas with sliced avocado.