There's a huge difference. In general, the idea is that we have to pay taxes to the state when we do business because the state facilitates it through creating an environment to make it possible. For a brick and mortar store, the state has made sure that the employees are getting paid a fair wage, that there are utilities provided on site, that the area is zoned properly, that crime is low which allows people to travel to and from the business, that there is recourse for theft on the part of either party, that claims made about products are true, that the products are safe and if someone gets injured there is recourse, etc, etc. For online sales, all the state has to do is make sure the mail gets delivered.