For the pomegranate syrup: In a small heavy saucepan, combine the pomegranate juice and sugar and bring to a simmer over medium-high heat. Reduce the heat to medium and cook until the mixture is thick and syrupy, about 30 minutes. Remove from the heat and allow the mixture to cool completely (the syrup will thicken as it cools). Stir in the lemon juice. Transfer the syrup to an 8 1/2-ounce glass jar with a tightly fitting lid. Refrigerate.
For the soda kit: Package the syrup with the seltzer and pomegranate seeds and include the following recipe card:
Homemade Pomegranate Soda Recipe
Place 1 tablespoon pomegranate syrup at the bottom of four 8-ounce drinking glasses. Top glasses off with seltzer water and gently stir. Add ice cubes and garnish with pomegranate seeds.