I'm waaayyy behind today - so I'll take the first part of the question now and post the legal reference here a bit later this evening.
Walking in a personal supply of anything will usually get you fair treatment regarding the law. You're there. If you know the law you can quote it, challenge it, etc. So personal use quantities of anything legal shouldn't cause difficulties when you're crossing a border - any border (not including libya at the moment though... lol). That said - I've also seen customs agents do very weird, illogical things at borders. I remember one time I brought in a pet from another country. One agent stopped me and wanted a bunch of papers I didn't have and knew I didn't need. I think he was about to impound my little furry friend when another stepped in. He showed something to his colleague who, it appears, realized he was in error, and let me and my pet through. If I hadn't been there though - whole different story.
Parcels are different. Challenging the way they are handled is more difficult, time consuming and costly. You're not there and anything can happen. Even the most innocuous items can be held for return by Customs. Brokers deal with that all the time. As vapers, we tend to hear about it more often because of what's going on right now with vaping supplies. BUT it can apply to anything from wrenches to dutch cheese.
