Oh, that’s not a nice message.
Sender can decide if they like to send the card in envelopes or not.
Some do it to protect the card, some to write more, some to add something with the card. Maybe to hide the message.
Don’t think too much of what they wrote. Probably they were disappointed at that moment, and wrote it.
But I also understand the receiver a little: part of the charm is to read what people choose to write in this small place. What do you say if you need to not say much? For me it’s a challenge
If and only if you feel like it, you can offer to send them only a card, without envelope, with hand written note in it. Then they are happy too, and you can show your intention was good. Maybe they will send a card back to you then
Because, elsewhere I sent a card, that was painted using toes, I like these cards for some reason, and it’s very cool. The receiver thought it’s disgusting to think someone used their toes, and I should send normal cards. I thought, ok, it can feel so, and told, I can send a card that is not like that. But they said, no need to, and apologised their reaction, they were for some reason angry, didn’t like the idea of toes. But we swapped cards anyways.
I have learned: if the card doesn’t feel good, I wait until next day, so I don’t write my instant bad reaction.
If someone is disappointed, and still seems like somewhat normal person: I offer sending something different - but this is my choise, and I only do it when I feel like it, and not to please anyone. (Like the toe painted card - when I thought about it, yes, someone can find it strange and even disgusting.)
But, don’t stop because one person writes like that