At what point do I have to register my company in another state as a "foreign" company?
I own a NH LLC. If I travel to VT to demo a product for a customer, does that rise to the level of doing business in VT so that I have to register with VT (and pay fees and taxes)?
Do the rules vary by state? Is there a general rule? Does anybody have a reference to a list that collects which states have special requirements or exceptions?
In my experience, the states do not make this obvious and won't even necessarily tell you if you call them. But California, for example, would consider you to be doing business there if you visited even for one day to do business.
These are state laws so they do differ from state to state and there is no general rule. They usually post the rules on their website but after you read a few of them you'll see they are not that helpful.