Buy something online or by mail order and you don't have to pay taxes on it, right? Not so fast. If the purchase would normally be taxed if bought at a local store, you are supposed to pay tax directly to the state.A 72-year-old Chaska woman called Whistleblower to complain about a letter she received from the Minnesota Department of Revenue telling her to calculate and pay back taxes on 11 years of untaxed tobacco purchases. "[I] didn't know it was against the law. It's hitting me especially hard," she said.The department said it sent out 1,100 such letters recently and said it works with "a variety of reliable sources to promote compliance with cigarette tax laws," spokeswoman Lisa Erickson said.Taxpayers with questions can call 651-297-1882.