It's not voluntary, it's the law. Pretty much every state with a sales tax also has a "use tax." Under the terms of this law, residents are supposed to pay the state in which they live the difference between the sales tax paid at the point of purchase and the tax due in the state where the product will be used. This also applies to mail order goods. In other words, just about everybody has broken the use tax law at one point or another because nobody pays volunteers to pay the use tax. This is why the states want to make mail order houses collect the taxes for them.
A few years back, California started getting customs records and sending use tax bills to people who returned from abroad with expesive items. It was not a popular move.
So if I buy something from New York at a sales tax rate of 8.5% and bring it back to use in New Jersey which has a sales tax of 6% can I send NJ a bill for the difference?