A postcard was a type of written correspondence.
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)
Samantha 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)
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)