Combine the lime zest, lime juice, olive oil, garlic, chili powder, red pepper flakes, cumin, and salt in a small bowl. Transfer to a large resealable plastic bag; add shrimp and toss to coat. Refrigerate for 30 minutes. Remove shrimp and discard marinade. Thread 3-4 shrimp onto metal skewers.
Heat an indoor or outdoor grill to medium-high; oil the grill grate. Grill shrimp for 2 to 3 minutes a side or until they are pink.
Place the lettuce from the Fresh Express® Twisted Avocado Caesar Chopped Salad Kits® into a large bowl; toss with half of the salad dressing and all of the toppings.
Divide salad evenly among tostada shells. Top each salad with 2-3 shrimp; drizzle with remaining dressing.