The formal version of "謝謝", "對不起", especially in an email? In English, when writing a formal email, one would sometimes use appreciate/grateful and apologize in place of thank you and sorry, respectively. Is there any such alternate words/phrases in Chinese for 謝謝 and 對不起?
For instance, how can the following be rephrased in a formal tone?

對不起現在才回覆您。
謝謝您的幫助！

Thanks in advance!
Alter words:

谢谢：多谢，感谢，谢恩（古代臣子百姓对君王说的）

对不起：抱歉，有愧，歉仄

As your case, 

對不起現在才回覆您。

謝謝你的幫助！

rephrase:

（非常）抱歉现在才回复您。

（十分）感谢您的帮助！