Pop and non-'healthy' fruit drinks are sold by the liter here. 'Healthy' fruit drinks are sold by the quart, never liter. Smaller pops and recreational beverages are sold by the ounce, unless you want a special drink, which will be a single liter. Pop is never sold by the gallon. Milk is never sold by the liter.
If you ever say you want a gallon of pop or a liter of milk, you will be considered weird.