Just because you don't have a physical presence in the US doesn't mean that you don't have a nexus. This is because Bellas Hess and Quill where both tossed out of the Supreme court in 2018. You can definitely have an economic nexus in the US. Subscriptions or software might need to be charged sales tax in many states where there are over 200 sales transactions OR over 100,000 in gross revenues derived from a state in a rolling year. Even if sales tax isn't charged there might be a use tax notification requirement that lasts for a full year for one transaction. Also you may need to open a foreign corporation in some states that you are conducting business in and pay income tax in those states as well and pay the minimum franchise tax.
For example in the state of Washington, USA:
Software might be charged sales tax:
Information technology products and services | Washington Department of Revenue
It applies to remote sellers with no physical presence in WA: Remote sellers | Washington Department of Revenue
The minimum fine for completely not following WA use tax notification law is $45,000: Use tax notice and reporting penalties | Washington Department of Revenue
