In a microwave safe bowl, combine the juice and agar and whisk until incorporated. Microwave for about 3 minutes, until boiling. Place in the refrigerator until lukewarm.
When cooled, add the champagne and agave, then whisk to combine.
Pour the mixture into individual 2-ounce shot portions or onto a lined baking pan. Chill for 8 hours or overnight until firmly set. If using a baking pan, use a pizza cutter to create squares and lift with a spatula. Store on parchment paper. Shots stay good for about a week so these work well to make in advance.
Notes
If you are using agar-agar flakes, the amount to add may be different so read the label to assess.