Postcards offer some of the most interesting rates and single usages that you can find. Airmail cards could be sent cheaper than the normal rate and there was usually a stamp designed just for that purpose in most issues. Used singly they are usually scarce.
The postcard rate also applied to printed matter which included printed cards. These cards were allowed to be posted at the lower rate provided they were sent in an unsealed envelope and contained not more than 5 words of additional writing.
So you could send a Christmas card to Auntie Vi in England in an unsealed envelope with a couple of words on it at the cheaper rate.
Here is a nice example of the 18c Fauna Conservation used for exactly that purpose. The envelope is unsealed and the backstamp is partially under the flap proving that it was never sealed. This is a scarce usage.