Tricky one. I'd say this. A package will be something that's been wrapped up, and very likely something that's then sent somewhere, for example in the post. The word refers to the whole thing - the thing that's sent - not to the thing inside it. Pack can be short for "back-pack" meaning a rucksack. We also speak of a "pack of cards", but that's perhaps a slightly different meaning. The letters "et" suggest something smaller. So packet suggests a smaller pack. But "packet" is in fact in rather the more common use than "pack". Typically a packet would be a cardboard box with perhaps some food item in it which you would buy in a shop - eg a packet of cornflakes, a packet of tea. If you wanted to, you could say a pack of cornflakes, but packet would be more common I think.