询问关于网购退税的疑问！

kelly926

在amazon买了一个相机。跟他们要退税发票，没有。但是他们教给我另外一种退税的方式。

  在信中，他们说：
     My colleagues of the payments-department informed me that you have
to go to the customs with your order (goods) when leaving the
country.
    You will receive an statement (a kind of invoice) of the goods which
are being abroad. Then you send this statement together with a copy
of your passport to us。

   我不知道怎样才能在海关拿到一个statement之类的东西证明我的物品已经出国了？
麻烦解答，不甚感激。