The printing company should be able to supply you with a template of the packaging net for you to work from, which you would then supply as a single piece of artwork.
Working with packaging straight on the net can get tricky though as you can (depending on the packaging) have all the different faces at different orientations, so you end up having to do half your artwork upside down and some at 90 degrees. It's more trouble than it's worth...
So what I normally do is create a page for each face (usually tabs and the like would be kept as part of the face they are attached too), at the correct size with full bleed. Then you export those pages and place them correctly in the template (masking any bleed that isn't needed). Doing this at the beginning and placing them as linked files means you can carry on working on your single pages and see the whole net update as you re-export the pages.