South Carolina customers of Amazon.com are receiving emails this week reminding them to pay taxes on all the purchases they made from the online retailer last year.

While customers may be surprised, South Carolina residents have always been required to pay sales tax on online purchases — it’s just a question of who collects the money. Retailers that have websites and stores in the state, such as Walmart, Lowe’s and Barnes & Noble, must collect tax on sales to South Carolina residents, but Amazon.com and websites with no physical business in the state don’t have to collect the tax.

Amazon fought a hard public battle last year to keep from having to collect South Carolina sales taxes, after announcing plans to build a distribution center in the state, and received a 5-year exemption from the state. As part of that agreement, the company was required for the first time to notify customers of the responsibility to pay the tax.

The emails from Amazon tell customers how much they spent last year, and how to pay the tax. The easiest way for South Carolina residents to pay tax owed on online purchases is by declaring the amount on their income tax form.

State residents appear to have largely ignored the law in recent years — fewer than 12,000 income tax returns included such tax payments in 2008, the most recent year for which data was available. Amazon is not required to furnish the state with information about customer purchases, according to the Department of Revenue, so the state has little ability to enforce collection of the taxes that Amazon’s customers are being told they owe.