Heat oven to 350°F (or 325°F for dark or nonstick pan). Grease bottom only of 13x9-inch pan with shortening and lightly flour (or spray bottom with baking spray with flour). In medium bowl, mix pretzels, sugar and butter. Sprinkle evenly on bottom of pan; press gently.
In large bowl, beat cake mix, margarita mix, oil, 1 tablespoon lime peel and the egg whites with electric mixer on low speed 30 seconds. Beat on medium speed 2 minutes, scraping bowl occasionally. Pour batter over pretzel mixture.
Bake 35 to 40 minutes or until light golden brown and top springs back when touched lightly in center. Cool completely, about 2 hours. Frost with whipped topping; sprinkle with additional lime peel. Store covered in refrigerator.