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".
Then it's context. If you're going for the excited delivery, try throwing in a ! or a "That's" beforehand otherwise it could be misinterpreted as uncaring or dismissive.
To be honest I think it's the trailing full stop that does that. Over text I find that usually comes across like you're trying to shut down the conversation (even when you're not)
I've heard from Gen Z that ending a text response with a period is passive aggressive. So "cool" may have seemed glib on its own, but the punctuation might be giving an additional, if unintentional, tone
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.
You need to find ways to convey your intended tone of voice, that's just the way it is. You could add a smiley. Even "Cool!" already looks a lot more enthusiastic than a dry "Cool." with a full stop.