How can I estimate average shipping costs and set up conditional free shipping?

[yaseminerdinc]

I will be selling affordable jewelry on my store and most likely planning to use USPS (First class mail and Priority Mail) as my shipping service.

1. How can I know how much on average the shipping cost will be for me considering I'm planning to ship to all US from NY?
For example shipping to NY (from NY) is cheaper from shipping to Los Angles from New York. Hence I want to understand how I can predict the average price and decide which service I would like to select.

2.I'm having difficulty understanding how I can link a conditional shipping (for example all orders > $100 will be FREE Shipping) with for example USPS Priority Mail shipping. How does the system understand that it will use USPS Priority Mail shipping and charge the customer $0 if their order is >$100?

Thank you very much,

Yasemin