Plugin WP e-commerce – shipping and stock query/help!!

Hi, I’m new to this so please bear with me! Some of these I think I just want reassuring I’ve understood, but I’d appreciate some help please!!

I’m setting up an e-commerce shop on my site, using the plugin from instinct entertainment, and I have some questions about the shipping settings and stock settings.

I’m selling unique jewellery pieces, and so I generally only have one of each. In the Product details section, when editing the product does setting the option “I have a limited number of this item in stock. If the stock runs out, this product will not be available on the shop unless you untick this box or add more stock.” mean that if I specify I’ve only got 1 then someone won’t be able to try to buy 10 of them?

My major problem is I’m not sure how the shipping works. It seems like I can set shipping in the plugin setting, AND in the product setting. Which seems rather silly as I don’t want people to pay twice! But I also don’t want to end up only getting £1.50 to post something to Australia. So:
1. which setting should I use, the product specific one, or the overall shipping setting?
2. How do I set it so places outside the UK cost more to post to?
3. If someone buys three things from me, how is the shipping worked out? If I set up the overall shipping setting to go by weight does the shopping cart add together the weights and calculate the cost from there?