A couple of things come to mind from this post - first, I've been curious - what heat pack was used?
You only give a guarantee if one as requested? Sad.
And you have the nerve to say she should be a better shopper? That pretty much makes you an

, and I feel pretty secure in saying you've lost a bunch of potential customers. Sure, it SHOULD have been there in one day, but it wasn't. That may not have been your fault (aside from not being aware of conditions), but that doesn't relieve you of your responsibility to your customer. You owe her a refund.
It's pretty simple - if you aren't willing to guarantee live arrival outside of a certain temp range...don't ship outside of that range. And if you are going to hide behind as statement of carrier delay or error, then maybe more people should rethink their purchases (as you said, be better buyers).