Translation:I put the postcard in the mailbox.
ハガキ is actually one of those random words that for some reason is written using katakana quite often, despite it not being a "foreign word". It's very probable this is done to improve readability, since it's so easy to mistake a は in the middle of a sentence as the topic marking particle rather than the first syllable of a noun.
They use all three ways of writing it, but it IS usually in hiragana. The thing with the katakana is that they are sometimes used for common hiragana words. They get written using katakana for extra emphasis and/or easier identification, so you are probably right. In some sentences that might be a problem.
Funny enough, they also use ポストカード too? I get very similar image results when I search for each term, though ポストカード seems to show more souvenir or graphic cards, and perhaps ハガキ might be more of the everyday type? I'm not sure about that.
This page http://www.cjvlang.com/Writing/writjpn/signs/hagaki.html explains. "The motivation for using katakana is possibly a desire to give the word greater 'standout'." (...than the hiragana, which are primarily for grammatical elements...but are simpler than the Chinese characters which have so many strokes) Real answer: it's common to do so.