Premix and postmix are two methods of serving soft drinks—usually carbonated—that are alternatives to bottles and cans. Premix means that the soft drink already has the syrup and water mixed. Postmix means that the syrup is separate, and is mixed by while the drink is being dispensed.
References
Mark Pendergrast (1993). For God, Country and Coca-Cola. New York.{{cite book}}: CS1 maint: location missing publisher (link)