I have read in a few blogs / articles about the need to pay taxes on income from different US states where my customers may be located, and therefore establishes some sort of nexus with that state. However, what is confusing to me is how should I collect / pay tax for every single state even though I have physical presence in the USA. For instance, customers use my SAAS app, and pay me a monthly recurring fee. This is handled by the payment gateway. However, my SAAS is registered in another country (like Singapore). The question is do i have to declare / pay taxes in the USA even though my company has no "physical" presence such as office, employees etc? Thanks.