Pls correct me if wrong, thx

首先提點一個英文習慣，第一身(I)寫信或直接跟別人溝通時甚少會用past tense的(雖然件事係在過去發生了)。第二留意數量的用詞”a” xxx board(s)是錯的配搭，a就只會一件board；從你的內容不能確定你是錯送了一塊還是很多(第二句寫了samples又是多件，所以先寫作多件，唯有你自己修改了。第三，用詞問題：surplus是指有先預設生產量而多產的才叫surplus，你估計你意思是意外地額外多加或多送了的不是叫surplus，通常會叫addition或extra。第二句中的our controls samples不太明白你的意思，我估你是想解釋你公司要控制樣板發送的數量或類別，但這是你的內部行政/錯誤，不是對方要體諒你的理由。其實簡單講「請把那些extra退回」便可以了。你第二句表達「假如你們找到…」好像不太確定你真的送錯了，我是對方便回覆「we do not find any extra samples in the box.」。另外商業上由於是你承認send錯，那送回來的費用誰付呢亦應該要先註明，假如是現有客戶還可以扣數，但如果仍未有採購合約則對方為何要花錢寄回給你。修改句式的寫法請自己看吧。 I have made a mistake in packing extra sample boards in the box that was sent to you (yesterday). I appreciate if you may send the extra items back when you’ve received them. Please accept my apologies for the inconvenience caused.

I had packed an extra sample board by mistake, and had already sent out the package to you. Please return the surplus control sample if you happen to find it.

Mistakes and Corrections
"I made a mistake for packing a surplus sample boards in the box to you.
Please help to resend to us for our control samples if the samples are found in your place."

There are just a few mistakes. My suggested corrections are in bold type below for your reference.

I made a mistake in (for) packing a few surplus sample boards in the box we sent (to) you.
Please help to return them(resend ) to us for our control samples if they (the samples) are found at (in) your place.

Dear _（那人的名字，如不知道，就寫 Sir / Madam）_,

I mistakenly packed some surplus（其實是否硫磺 sulphur？）sample boards in the box sent to you. Please kindly help send us back these control samples of ours if you find them in the box. Thank you so much for your help.

Kind regards,
__（你的名字）__

2013-07-29 15:29:00 補充：
第二句客氣點的話，可改成:
I would be grateful if you could help send us back these control samples of ours when you find them in the box.