Two questions from newbie in US business

Hi,
I am currently setting up a new corporation in Cheyenne, Wyoming, and I will run it from Denmark, where I live. I am danish and this is my first time setting up my own business in the US.

I also have a web shop, and I intend to sell products in the US through my US business. Is it correct that I can sell to the entire US from a company only registered in only one state?

I need to look into customs and I have no experience what so ever regarding US customs. Do I need to look at national authorities sites and rules or is that a state matter for Wyoming? Where do I start?

Thanks,