The customer must file a use tax return and pay the tax if the goods or services were purchased for use in your resident state and if any tax paid to another State is less than your state tax rate.
For example: Suppose that you purchased a $1,000 refrigerator on October 1, 1991 in Massachusetts, where the tax rate is 5%, and you paid a $50 Massachusetts tax. If that refrigerator was purchased for use in Connecticut and CT was your resident state, then a Connecticut use tax is owed. The Connecticut tax of $60 is reduced to $10, after allowance of a $50 credit. If no Massachusetts tax is paid, the Connecticut tax is $60.
This is the way the rule is written, now reporting it and the paperwork is an entirely impossible task.... Most just estimate or only pay for large ticket items such as cars and untaxed internet purchases.