Is the following sentence correct? I want to translate to Chinese this phrase: "Please, write on the parcel  "some-text" ". 

Is this variant correct? 请在包裹外面上写下 "some-text".

Thank you.
We don't usually say“外面上”，instead，we will say “包裹外面” or just “包裹上”, and they have the same meaning here.