How to shop for back-to-school tax-free

If you're going to drop hundreds of dollars on back-to-school supplies, you might as well do it now.

Starting this week, 15 states are offering tax breaks that can save shoppers between 4% and 7%. (Mississippi had a tax holiday in July).

Families with kids in K-12 will spend an average of $669.28 prepping for back-to-school this year, according to the National Retail Federation, so that kind of savings can really add up.

The breaks tend to apply to clothing, computers, software and school supplies, with the "holiday" usually being a two- or three-day period at the beginning of August or a longer sale toward the middle of the month.

But shoppers can't run out and buy lots of designer duds or high-end Apple computers, since most of the state tax holidays have restrictions. Most don't allow tax breaks on items that cost more than $100, according to Carol Kokinis-Graves, senior state tax analyst for Wolters Kluwer, CCH, which released a report on the holidays. Some exclude sports equipment and jewelry.

Also, while you'll get a break on state taxes, local taxes can still apply.

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Those limits are why Lyman Stone, an economist at the Tax Foundation's Center for State Tax Policy, said he doesn't think the holidays do much to boost a state's revenue growth.

He said studies have shown that the holidays only shift when families buy goods, not how much they spend. He adds that retailers sometimes fail to apply the tax break or simply try to get around it.

"All these extra people go to the stores, so stores can afford to raise prices a bit," Stone said.

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The good news is that you can usually shop online and still get the sales tax break, although you may have to pay up for shipping to ensure you take delivery before the end of the holiday period.

If you're looking for some stand-out deals, a few states offer sales with fewer limits.

In Missouri, shoppers can spend as much as $3,500 on a computer tax-free. Louisiana has a lump-sum break on $2,500 in spending, and South Carolina's list of non-tax items includes bed linens, shower curtains and other dorm room goods.

If they look enticing to you but you don't live in these states, feel free to hop in the car: Kokinis-Graves says she doesn't think she has "ever come across" a residency requirement.