中譯英... business letter

首先講講用詞
mock-up是樣版/樣板(物品)或者是示範單位(建築) = 一般講mock-up只是有外觀型態而沒有全部的內部設備的。例如汽車或戰機生產商都會製作mock-up實物大小的模型給商家/"老闆"檢驗是否跟構想圖相同或合不合意，但未必是用真正的物料或結構，亦沒有發動機或操控系統等。

但你的主要意思不是單單做個樣版/樣板，而是真實「生產」的模板成本，所以我會直接講出這是討論真正生產的"mass" production最終的實物產品。由於這用詞是專業範疇，所以上特別指出這點你可多多留意(你可以參考一些專業的同行如何用詞或生產工序上的分類用詞)。

其他用詞或英語的文法寫法請自己參考以下修改。

We can customerize the USB Flash Drive as you have requested, but I would like to know your quantity of order. It may be expensive to produce for small quantity that may exceed the standard pricing.

Alternatively, you may consider to use our model (COM062) to adapt for your own design.

[用to adapt是假設你的意思是用com062的殼來印上他們的設計圖便可以了-但假如你的意思是可以改造個殼的式樣則可以用to modifiy才表達出更多的空間給你容戶按他的設計作出決定]

Thanks for your email.

We can customize the USB flash drive as you request. But I would like to know
the quantity you would order. If the quantity is too small, it is expensive to
mock-up. Therefore, I would suggest on an alternative to make up your
design - convert our existing model (COM062), using its shell for your new model.

Looking forward for your reply !