A postcard was a type of written correspondence.
History[]
Samantha Briggs discovered that Chameleon Tours gave out stamped postcards to its young airline passengers for them to write before they departed, which were later sent abroad and posted back to England so their families would believe they had arrived safely. She realized her missing brother, Brian, wouldn't have had to be in Rome to send the postcard she received from him. (TV: The Faceless Ones)
Duggan bought a postcard of the Mona Lisa from a Parisian gift shop. (TV: City of Death)
While visiting Ostend with the Sixth Doctor in 1913, Peri Brown was going to send H. G. Wells a postcard. (AUDIO: Year of the Pig)
When Midge went to Australia on holiday, he sent Ace a postcard saying how lonely he felt so far from home. (PROSE: Timewyrm: Revelation)
Sam Jones sent her parents postcards from the planet Kursaal and one of her and the Eighth Doctor in a photobooth on Paphos. They started arriving around the Mars, 1997 mission. (PROSE: Unnatural History)
The Timey-wimey detector was made up of items including a Blackpool postcard. (TV: Blink)
While on Vallarasee, Donna Noble asked the Tenth Doctor whether or not she could send her grandfather a postcard from the 51st century, to which the Doctor did not know. (AUDIO: One Mile Down)
Brian Williams visited Siluria and sent back a postcard to Rory and Amy. (TV: Dinosaurs on a Spaceship)
Physical postcards still existed in the 27th century, which surprised the Twelfth Doctor. (PROSE: Big Bang Generation)