You could say, "The postcard was a feature of the British summer holiday for over a century" - that is talking about the general idea. In your sentence you are talking only about postcards you received or sent.
I've learned that "A Noun", "The Noun" and "Noun s" can all refer to the "Noun" in general. For instance:
A tiger is a dangerous animal.=The tiger is a dangerous animal.=Tigers are dangerous animals.Both are possible.By the way, should I say "my holiday" or "my holidays"? I mean "holiday" in general.