Paribus When Gus and his wife bought the outdoor sofa set of their dreams for $1300, they couldn't believe their good fortune.

They had found it for $1000 less than they had seen anywhere else, and then had a $600 coupon they could use towards their purchase. What a good deal, they thought!

But then came the best part: Notice about a rebate of nearly $300 arrived in their inbox less than a week later.

This is the kind of story that makes 25-year-old Eric Glyman smile.

Gus and his wife got their big-ticket reimbursement thanks to Glyman's newly launched startup, Paribus.

The company's motto: "Stores owe you money. We're gonna get it for you."

Paribus takes advantage of the fact that many of stores vow to refund customers the difference if their competitors offer the same product for cheaper or if they introduce their own additional discount not long after the initial purchase.

There are two barriers to people actually ever getting any money back though: Every store has a different specific policy, often buried deep within the fine print on their websites, and most people don't want to go through the hassle of keeping tabs on dropping or competitor prices, much less painstakingly contacting the company in question.

As a teenager, Glyman worked at Express, and distinctly remembers watching families come in on Saturdays to buy thousands of dollars worth of clothes at a 20% discount. By Monday, the same items would get slashed to 40% off, but very few people came back to redeem the refund Express technically owed them.

The breaking point to his frustration came much later, when he and a group of friends bought plane tickets for a vacation, and the next day, another friend decided to join and he found tickets for $100 cheaper. He started researching the different ways companies allowed people to get refunds, saw how complicated it was, and decided to try to build something to simplify the process.

"It shouldn't be so time consuming — people shouldn't have to spend more than half an hour for $4 back," Glyman says.

Karim Atiyeh and Eric Glyman Paribus In summer 2013, he and his friend and fellow Harvard alum, Karim Atiyeh, decided to tinker with the concept and the tech.

A year later, they decided to quit their jobs to focus on the startup full-time.

Paribus launched in beta in September. Users sign up with any email address that they plan to use for most of their online purchases (Glyman says many users opt into making a completely separate account).

Every time a receipt hits their inbox, Paribus scrapes the product information and will spend several weeks poking around for potential discounts. If it finds one, the shopper will get a refund.

Paribus takes a 25% cut of that rebate when it saves you money — though the first one is free. Right now, the company will let anyone who invites five friends spend their first year of the service not giving it any of their cash.

YCombinator recently accepted Paribus into its summer accelerator program and the service currently has more than 10,000 subscribers. Although most people don't get the slum-dunk payouts that Gus and his wife did, those kinds of savings do happen. More often, shoppers end up saving smaller increments — $5 here, $20 there. Several members have saved $500 in the past two months, others are perfectly happy with $15 over the same time span, because they're getting it through virtually no work on their part.

That's the best part about Paribus, sign up once, and from then on you'll just watch money you never would have gotten otherwise flow into your inbox.

Although Paribus hasn't received any official contact from retailers like Amazon or Best Buy that it provides refunds for, he argues that his business is good for their business.

Amazon is all about customer satisfaction, but its customers can't possibly monitor its 80 million price changes per day. When shoppers can get their money back easily, it's good for Amazon's brand.

"We're out there, we're running super fast," he says. "And we love doing it."