There is a Customs Declaration form number that you get when you fill it out in the post office, but that's pretty much all you can offer to give to your customer when it comes to international shipping. Also, 2 weeks doesn't mean she will not receive it. It can take way longer than that sometimes, because some items get stuck in the customs. So, if you have the Customs Declaration number give it to your customer, and inform her that international shipping can last longer than two weeks, and ask her to be patient because she should get it soon. I'd also tell her that if she doesn't get it, you will refund her money, or re-ship the item... whatever she prefers. That should calm her down.
Best wishes,
Ivana