Please check the grammar, thanks

你的句子應改為：
The original invoice has been sent to you by mail.
had been sent是過去完成式(past perfect tense)，其基準時間為過去的某定時，如：
I didn't know who had broken the window.
had broken the window是在didn't know之先，也已作者didn't know這動作的時間為基準時間。
又如：
He had been ill for a week, when the doctor was sent for.
had been ill是發生在the doctor was sent for之前，也以the doctor was sent for為基準時間
你大可以看出，過去完成式的基準時間都是過去的時間
而現在完成式(present perfect tense)，是以現在為基準時間（不是我的現在，而是說話者的現在），如：
I have not seen you for a long time.（其基準時間，就是說話者講”I have not seen you for a long time”的”現在”）
你的句子既沒有另一個過去的動作（時間）為基準，故要用現在完成式

希望幫到你！

1st snetence
'had been sent' is past perfect tense. We use it to talk about sth happened in the past, eg. I don't know who had eaten the cake.
That is eaten the cake come first ,then I didn't know.
Here we use ''eaeten the cake'' as a time reference
Therefore we use past perfect tense.
2nd sentence
'has been sent' is present perfect tense. We use present perfect tense to talk about sth happened at poresent moment. Eg. I have given you a piece of bread. That means I give you the bread right now.
So we use "have given you a piece of bread " as the time reference.
Therefore, we use present perfect tense.
As I saw your sentence hasn't got an action to be in the past, so I think present perfect tense is more suitable
P.S. Both sentences are correct, no gramatical or agreement mistake.
Hope I may help you.

The original invoice had been sent to you by mail.
文法正確，
意思通順，
完全沒有問題哩
圖片參考：http://tw.yimg.com/i/tw/ugc/rte/smiley_22.gif


2008-06-06 09:08:15 補充：
had been sent 同 has been sent 的分別就係︰
雖然兩者的意思都係已寄出，
前者並沒有說明幾時寄出，
可能係一年前或一個月前，
但後者係present perfect tense,
表明係剛剛寄出，
正就是同前者的past perfect tense的分別了，
希望你明白。