When you’re out doing your shopping, you’re always looking for the best price.

But what if the best price automatically came to you — even days or weeks after your purchase online or in stores?

Welcome to the world of apps and services that relentlessly search for better prices on what you bought and make retailers pay you the difference!

RELATED: Should you register your new products?

Put money back in your pocket without lifting a finger!

You’re probably familiar with the claim most retailers make that “we will not be undersold.” It’s a classic retail ploy to assure you that its okay to buy because if you find a lower price, the store will beat it — if you remember to go back to the retailer with your receipt and proof of a lower price, that is!

But for most people after they buy something, it’s out of sight and out of mind.

Almost nobody still combs through the sales circulars looking for a lower price on something they bought last week and already handed out as a Christmas gift, right?

That’s where the power of technology comes in.

Using specially developed algorithms, the following free services can put a little extra jingle in your pocket when you’re playing Kris Kringle this year.

Citi Price Rewind

Here’s a free service that’s available to all Citi cardholders!

The savings start when you register the products you’ve purchased with your Citi card at CitiPriceRewind.com.

Once you registered your purchases, Citi then combs through millions of listings using proprietary software to look for better prices on your buys for 60 days after the date of purchase.

If they find a lower price, Citi will email you so you can request the price difference from the retailer. You can get up to $500 back per item, with a $2,500 limit per year.

Visit CitiPriceRewind.com for more details!

Here’s how much this free service has paid out this year so far…

Paribus

Paribus could be the next best thing to finding free money.

Unlike Citi Price Rewind, this free service is open to anybody and you don’t even need to lift a finger to claim a refund if you’re told you’re eligible for one!

The money-refunding fun begins by connecting your inbox to Paribus so their Receipt Fetcher can identify your purchases. Any purchases it finds are then cross-referenced against a growing list of retailers that its proprietary software is compatible with.

If a lower price is found, Paribus automatically files a price adjustment claim on your behalf and you get to keep all of the savings!

Here are the retailers Paribus currently works with:

Big-box retailers

• Amazon
• Costco
• Kohl’s
• Sears
• Target
• Walmart

Apparel

• Anthropologie
• Athleta
• Banana Republic
• Bloomingdale’s
• Bonobos
• Gap
• J.Crew
• L.L.Bean
• Loft
• Neiman Marcus
• Nordstrom
• Old Navy
• Saks Fifth Avenue
• Zappos

Housewares

• Bed Bath & Beyond
• Crate & Barrel
• Home Depot
• Overstock
• Wayfair

Electronics and office supplies

• Best Buy
• Newegg
• Office Depot
• Staples

Sporting goods

• REI

Yapta

Have you heard of airfare envy? That’s when you buy a ticket and find the same itinerary for less money later on.

The good news is that there are a handful of airlines that will give you a voucher for the difference if you ask. These include Alaska, JetBlue, Southwest, United, US Airways and others.

But unless you religiously follow airfares after you’ve made your purchase, you may not even know that you overpaid. That’s where Yapta.com comes in handy.

After completing the free registration and detailing your itinerary, Yapta will e-mail you when your flight goes down in price and help you get a refund for the difference.

The average annual savings you’ll get is $334.16, according to Yapta’s website. The free service has saved members more than $250 million since launching in May 2007.

RELATED: Love a good deal? Here are 4 ways to save even more money at Walmart

Shopping online? Here’s how to spot fake sites

[anvplayer video=”4116309″ station=”998267″]