When I started my online store, one of the things that I did not expect to prove as difficult as it did was figuring out and generating shipping quotes for my users. This is so difficult because shipping is not as simple as you might have thought that it would be. You could just charge customers a flat rate for shipping as long as they live in the US, but you will still need to offer different shipping priorities, and additionally, you would be overcharging customers who live close to you – not only is that dishonest, but it lowers your price competitiveness versus other businesses.
There are some things you can do to make the whole ordeal a lot simpler, though. What I did was to go and download all of the shipping and price charts that I could from USPS, UPS< and DHL, and work them into a spreadsheet that would tell me which states and which priorities would cost which shipping rates. Now, I just have to consult my charts (here) to figure out what shipping rate to quote to a customer. You may also be able to get a USPS shipping liaison to give you the information.