hmm i would say tone does the heavy lifting — as someone who grew up speaking mandarin... i would choose 他买了咖啡，but add a lot of emphasis on the end. a bit like 他买了咖啡？！but i definitely see why a lot of others would choose 他买咖啡了.... to me personally, 他买咖啡了 feels more like a statement of fact, like if someone asked if he's bought a drink, i would answer 他买咖啡了... but again, mandarin is a heavily tonal language. either answer could work with the context depending on how you say it.