What’s considered a fixed office/permanent establishment address in the U.S. and will it be subjected to 30% withholding tax

My company is small and incorporated with no employees. I am required to get an EIN number to sell on Sears’ website. I also need a U.S. address if there are returns, which my sister-in-law has provided me with. Is it considered a fixed office (permanent establishment) address in the U.S.? Would I be subjected to 30% withholding tax because of that?