When multi-state coverage is actually required
The first question multi-jurisdiction founders ask is also the most confusing: when is registration in another state actually required? The answer is some version of "when you are doing business there", but each jurisdiction defines that phrase a little differently. The common triggers are an employee living and working in the second state, a physical office or storefront, significant in-person sales activity, and revenue thresholds that vary widely from state to state.
Plenty of online activity does not trigger registration on its own. A company that ships occasional orders into a state from a single home base usually does not need to register there. A company that hires its first remote employee in a new jurisdiction almost always does. The line between the two is fuzzy, and erring on the side of registering — when the cost is modest and the penalties for unregistered activity can be severe — is generally the safer call. Adding a new jurisdiction to your compliance footprint, paired with a competent statutory partner, is cheap insurance against being caught operating there without authority.