Sales tax nexus tracking across states for small ecommerce sellers

Small ecommerce sellers (under $5M revenue) who ship physical goods must independently track whether they've crossed economic nexus thresholds in each of the 45 states (plus DC) that impose sales tax, where thresholds vary ($100K revenue in most states, but $500K in Texas, $250K in Connecticut, and some states also have transaction-count triggers). So what? Once they unknowingly cross a threshold, they become legally obligated to collect and remit sales tax in that state retroactively. So what? They now owe back taxes plus penalties and interest on every past sale in that state where they failed to collect, and they can't go back and charge customers after the fact — the liability comes entirely out of their margin. So what? A single state audit discovering two years of uncollected sales tax on $200K of revenue can produce a $15K-$25K surprise liability that wipes out an entire quarter's profit for a small seller. So what? The seller either pays out of pocket (threatening cash flow for inventory purchases) or enters a payment plan that creates a monthly drag on the business for years. So what? Many small sellers simply avoid selling to certain states or cap their growth to stay below thresholds, artificially limiting their addressable market by 30-50%. This persists because each state sets its own thresholds, filing frequencies, product taxability rules, and registration processes independently, creating a compliance surface area that scales multiplicatively with the number of states — and no single free tool aggregates real-time threshold monitoring across all jurisdictions.