Large online retailers may have made their name by offering lower prices than their brick-and-mortar competition, but you still need to make sure you're snagging the best price.

Watch out for "the rule of three": Behavioral economy dictates that when given three pricing options, you are most likely to choose the middle-priced item.

Many retailers exploit this tendency, and will offer three different options in hopes of getting you to spend more, said Poundstone.

So if you find yourself drawn to the middle-priced option, stop and think about your original shopping goals and budget.

Try a second browser: Thanks to online "cookies" that track your web history, many online retailers know what you've been searching for or buying before you even visited their site.

In some cases, they may use this information to offer you a higher (or lower) price than is offered to other consumers, Poundstone said.

If you think this could be the case, try using a different web browser to double check the price before buying.

Try the abandoned shopping cart trick: Sometimes you can use those sophisticated tracking systems to your advantage.

Have your eye on a pricey product? Place it in your shopping cart. Then start the checkout process using your email address, but don't complete the purchase.

In response, some stores will attempt to entice you with a subsequent email featuring added perks, like free shipping or a percentage off your purchase.