when replying that way in any way, on text or actually just saying “cool”. my friends say cool a lot and I really don’t mind it and my partner says it too but some people have thought it was rude and I had a friend say “that’s what I say when I don’t give a damn”.
If it’s saying “cool” as an end to the conversation then it’s usually pretty rude. You need to show a bit of interest with a quick follow up like “how did you get it?” or “it took you a while?” and then ending the conversation is fine.