You are handling this with grace and I hope that your customer can see that.
I don't jump on the "they got what they ordered/the cake was fine when you left so you don't owe then anything bandwagon. Yes you made the cake they ordered, yes it was fine when you left, and yes you fixed the problem, but they did not get what they ordered and they do have a right to some compensation. (not aimed at the OP at all, just my general feelings on the whole thing) I hate seeing the posts where people are so up in arms because the customer is looking for compensation for a job that, while it started out well done, wasn't what they wanted in the end. If they didn't like the flavor they chose that's one thing, but to have a different cake than what was paid for... that is completely different ball of wax.
Hopefully they call back and are happy with your offer. I don't think that they are entitled to a full refund (or anything close to it) but they are entitled to something, and you are handling this so well.