"Upon" and "days before" don't go together. There are a couple of options for your sentence, depending on what you're trying to say: (1) Packaging details will be given upon shipment. This means that the details of the packaging will be given at the time the item is shipped. Another example of using "upon" in this context is "please contact the manager upon arrival", meaning please contact the manager at the time you arrive. (2) Packaging details will be given two days before shipment. This is probably self-explanatory, and obviously "two" could be any number.

'Upon' is a slightly old-fashioned variant of the simple word 'on'. In most cases when you meet the word 'upon' you can just replace it with 'on', for example, 'He relies upon you' means the same as 'He relies on you'. Your sentence above doesn't really make any sense, I'm afraid. Can you explain what you are trying to say?