Heat oven to 350°F (325°F for dark or nonstick pans. ) grease bottom only of 13 x 9 x 2-inch rectangular pan with shortening and lightly flour.
Mix pretzels, sugar, and butter in medium bowl. Sprinkle evenly on bottom of pan; press gently.
Beat cake mix, margarita mix, oil, lime peel, and egg whites in a large bowl with a electric mixer on low speed for 30 seconds. Then beat on medium speed for 2 minutes, scraping bowl occasionally. Pour batter over the 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 and sprinkle with lime peel. Store covered in refrigerator.