I forgot to write the Postcard ID on the postcard I sent

Don't worry, the receiver can still ask for help when they receive your postcard.

If you would like to make their life a little easier, just write them a private message to explain that they will be receiving a postcard without a Postcard ID, and include the Postcard ID in the message. Please note that the recipient can only register your postcard when it arrives in their mailbox.