When you cross a state's threshold, you usually have a limited window to register and begin collecting. In some states, the obligation starts on the day you cross or the first day of the following month.
Tax you should have collected but did not is still owed. You owe it from your own funds, plus penalties and interest. State penalties for failure to file and failure to pay commonly range from 10% to 25% of the tax due.
Alerting at 75%, 90% and 100% of each threshold turns this from a crisis into a calendar item. You have time to register, set up filing credentials and switch on collection before the first taxable order.
If you are already past a threshold, a voluntary disclosure agreement may limit the look-back period. Our VDA guide explains when that makes sense.