Or, looking at it the other way... if it's plausible that Comed-Tea is capable of influencing Harry to drink it at the right moment (or, rather, just before the right moment), then I'm not sure why it isn't plausible that the phrase "Wingardium Levioso" is capable of influencing objects to levitate.

Words don't normally have that ability, and I can't imagine how they could, but the same is true of soft drinks.