Preheat the oven to 350°F. Spray a 9×13-inch pan with baking spray.
In a large bowl, mix together cake mix, eggs, oil, soda, and strawberry gelatin until smooth.
Pour into the prepared pan and bake according to the box directions. Let cool completely.
In a mixing bowl, beat the butter until smooth. Add marshmallow fluff and mix well.
Add powdered sugar, strawberry soda, and extract. Beat until fluffy and smooth.
Divide the frosting into three bowls. Leave one white, tint one pink, and one green. Spread the white frosting over the entire surface of the cooled cake. Fill a piping bag fitted with a Wilton 1M tip with green frosting and pipe a basic shell border around the edge. Using another piping bag with a 1M tip, pipe pink frosting in angled “V” shapes (like a braided or chevron pattern) just inside the green border, layering in a criss-cross motion along all four edges.
Press strawberry candies gently into the pink piping to complete your soda shop look.