I think you were considerate to your customer for trying to save her money. Kudos on that.
But that being said, I think you should have shipped the package whole because the customer should be savy to what international charges are and she could have asked that you send it in two packages. I think it's the buyer's responsibility to know shipping and custom charges ahead of time.
Since she did not respond to your email in 24 hours, you should have shipped as ordered.
I agree with others that you shouldn't beat yourself up over this...you did what you thought was the right thing at the time, and there's something to be said about that.
Take care, Dani