Make each item cheaper when buying bulk and default shipping questions

Hi, I have an online store ( http://www.dunanova.com ) and I can’t find out how I can make it work so that if people buy in bulk they get cheaper price per item.
I am using wootique theme.

Here are examples

For example

1 item – $10
2 items – $18 ($9 per item)
3 items – $24 ($8 per item)
ect.

AND I need to be able to mix items that have the same price or are in the same catergory. But still are seperate items (not that you choose different colours/patterns from a drop down menu in the item page, but rather that when you put the items in a cart the price changes accourding to what you put in the cart).

for example

Blue shirt $10
Blue shirt and black shirt $18 ($9 each)

So that you could choose 10 different items and the price would automaticly change in the cart.

Do I need to add something to my site to do this?

Also, how can you make one shipping methond default but the buyer can still choose other methods

Hope someone here knows the answers, thank you in advance! 🙂

ps. how can I seach this forum, can´t find any seach.