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".
In my opinion, it is better to use 请在包裹上写下 "some-text".
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.
This expression can be understood by Chinese. But usually native Madrin speaker will express it like this "请在包裹上写下" or "请在包裹外面写下".