If you have not already done so, add the coconut milk to an ice tray and freeze.
In a high-speed blender, combine the frozen coconut milk, lime juice, tequila, Cointreau, agave, and 2 cups of ice. Blend on high until smooth.
On a small plate, combine the coconut flakes and tajin and stir to combine. Cut a slit in the lime wedge and rub it along the rim of a glass. Roll the edge of the glass in the coconut-tajin mixture to coat the rim. Pour the coconut margarita into the glass and serve with a wedge of lime.