Popcarte is a service for consumers, taking their uploaded photo and turning it into a card of the customer's choosing, from postcards to invitations to thank you notes, to special occasions, and then mails it out worldwide. The Popcarte API lets developers integrate the service into their apps.
More detailed information is available once the developer registers.
Popcarte is based in France, with websites in several languages across 8 countries. It mails cards out from the US and locations throughout Europe. Auré Kreed reports in Le Journal de Geek that the service is perfect when, "On the beach with your feet in the sand, you think of loved ones, or, on the contrary, you think of colleagues stuck in an office that you want to enrage with a picture of the magnificent sunny scene." (Author's translation.)
Postcards are printed 4x6 on 0.66 lb card stock in high definition. A single postcard costs $1.49; folded 5x7 cards are $2.99 plus postage. Cards are cheaper in quantity.