Soda Guzzling at Work....

So yeah, working as a barista is pretty chill most days. People order their usual drinks like they own the place, and I'm cool with it because hey, it's my job to make them happy. But there’s this one guy who orders an extra large soda every single day... and he doesn't even finish half of it! I’m tempted to just pour his leftover drink back into the fridge for someone else to enjoy, but obviously that would be nuts. Is anyone else dealing with weird requests like this? Am I overreacting, or should I just shrug it off...