Dear xxx,
I would like to thank you for being a valuable customer with us. And your unlimited patience to endure the mistakes I`ve made at work is most appreciated.
At the coming holiday season, I personally prepare a small Christmas gift for you and hope you will like them - a tea cup and a Chinese tea pack. And yet the gift cannot express enough my gratefulness for your generous support.
最後一句妳可以用夜未央的.
因為妳是要寫給外國人,所以我就用妳的意思按照外國人的寫法,而不是直接翻譯妳的句子. 妳的句子是中式表達,跟外國人表達方式不同. 還有,如果妳要送禮給外國人,最好找個正當的理由(like Christmas gift),有些外國人不喜歡直接收禮,因為那比較像賄絡(bribe).