Location Transport - What is the difference in USPS postal prices?

What is the difference in USPS postal prices?

I need to know the differences in USPS postal prices and codes. Like which is cheaper? Priority or first class?, What does flat rate mean? And what makes a difference on ebay when I offer free shipping? Thanks in advance....

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http://www.usps.com/prices/welcome.htm?from=home_shipstartfinish&page=compare http://www.usps.com/all/welcome.htm?from=home_header&page=allproductandservices

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