Postcrossing - Help
What is a Postcard ID and where can I get one?
A Postcard ID is a little code that uniquely identifies a postcard in the system. It is composed of two parts: two letters (the country code) and a number.
To get a Postcard ID, you simply request to send a postcard and the Postcard ID for that specific postcard will be given to you. This means only official cards have Postcards IDs (postcards that you have privately swapped with other users cannot be registered).
It is important that you always write in each postcard its unique Postcard ID - that is what allows the receiver to register that postcard when it is received.