I suggest implementing the feature: remark field for postcard
There shall be an additional field, where the account owner can put remarks to a postcard. This should be visible only to the author of the remark / owner of the postcard (if such a concept of ownership exists in the postcrossing system), i.e. the sender of the card or the receiver (whoever is the author)
The remark field may be used for things like “did use an envelope”, “put special stamp xyz on the card”, “got thank you card”, “did file the card in album 123” and so in. I guess the community will find/has a lot of ideas.
The remark field shall be visible and editable especially in list views like postcards “Travelling”, “Sent”, “Received”.
It would be great if that feature could be implemented and I will gladly provide more input if needed.