Estes dicionários dizem que o substantivo "postal" é o mesmo que "cartão postal":
http://www.priberam.pt/dlpo/default.aspx?pal=postal (Portugal). http://www.aulete.com.br/postal (Brasil).
I thought "a postal" sounded odd too, however, I typed what does postal mean into Google and was shown this:
That echos the definition in this dictionary: https://en.oxforddictionaries.com/definition/postal
I looked in a couple of US dictionaries but drew a blank, though perhaps it hints that Duolingo's answer isn't totally crazy.
My Living Language Portuguese Coursebook (admittedly 10 years old) asserts that "Contractions of de and em with the indefinite article (um and its other forms) are optional, both contracted and noncontracted forms being used: de um artigo or dum artigo → of an article; em umas aldeias or numas aldeias → in some villages" - although this contraction may well be mandatory in European Portuguese.
1- It's short for cartão postal and means postcard.
2- It means postal, as in postal services, the mailing service.
The second meaning is sort of global, but it's rarely used in conversation - I mean, it's not "rare" rare, but it doesn't come up that often (even in English it's not THAT common to say "I need postal services", right?)
But, regarding the first meaning, it seems there are some regional differences when it comes to spoken conversation:
- Where I live, in the south, we commonly use postal instead of cartão postal and it means postcard.
- According to other comments, some places know postal as anything you post (even a package), not just the card.
- And there's also some places where when you speak of postal, it refers to the mail service itself, not the things you send/receive.
Hope that clarifies it! If it doesn't, please say so C:
In the UK we do have Postal orders which is a way of sending money through the post, largely obsolete now I would think, but it used to be very popular years ago. 'Postal'seems to be a word which is more used as an adjective in English. I can't really think of any instances where we used that word as a noun, even fifty years ago when postal services reigned supreme! I am native English, incidentaly.