You are in the wrong.
The printer will print whatever size of artwork you supply. You supplied the wrong size, so it is your fault.
Also, postcard size is a different proportion from A sizes. It sounds like you didn't have a clue what you were doing, and are trying to blame others for the mess.
Why not take a little time to find out the sizes of a postcard, and supply it at the right size.
Obviously, you didn't steal photos and images from other photographers and artists, so there isn't really any excuse for sending the wrong shape image.