Combine the cranberries, sugar and 1 cup water in a medium saucepan over high heat. Bring to a boil and reduce to a simmer. Carefully smash the cranberries with a potato masher, then simmer the mixture until slightly reduced and dark red, about 10 minutes. Let cool for 10 minutes.
Strain the mixture through a strainer into a heatsafe container, pressing on the cranberry solids to extract all the juice. Let cool for 30 minutes.
Combine the cranberry syrup with the limeade and lime juice in a pitcher and stir well.
Mix equal parts of the drink mix and seltzer. Serve over ice with cranberries and a lime slice.
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