Under a certain volume of sales, it all depends on whether you have nexus in a US state. That means physical presence, which can be as minimal as having an employee or renting an office. Above a level of sales (varies by state) you would have to collect sales tax – this is the fallout of the 2018 SD vs. Wayfair US Supreme Court decision. Lastly, as you probably know, sales tax is state and locally administered in the US. There is no national level tax like GST or VAT. So it is very, very complicated (and annoying).