HOUSTON -

You're asking Amy about overcharges at the checkout, and she knows a little something about the subject.

She just wrapped up a year-long project we've dubbed "Operation Price Check." Investigator Amy Davis explains how much she was overcharged in 12 months, how she got the grocery store to pay for its mistakes and how you can do the same.

It all started when Davis was overcharged at her grocery store several visits in a row, and it ended with a decent little windfall for a deserving Houston charity.

The allure of a sales tag promising a 20-cent savings will work every time on a bargain shopper, but what if none of the shoppers who bite see that 20-cent savings?

It happened to Davis, and after a series of overcharges at her local grocery store, she wondered how much she was losing every year because of pricing errors.

Do you even stop to check your receipt?

"I always watch my receipts," shopper Mary Robinson said.

"I don't usually check it at the store, but I always check it at home," shopper Jennifer Lee said.

"I do look for the discounts," shopper Norina Silva told Davis.

For a full year, Davis checked her receipt every single time a cashier checked her out, and then she called them on every overcharge.

Sometimes she shopped with a hidden camera, and out of a total of 90 trips to the Kroger on 11th Street near Shepherd Drive in the Heights, she was overcharged on 23 trips. That means 25 percent of the time she shopped at the store, she was overcharged. She overpaid a total of $60.99. 

"At times, did errors occur? Absolutely," Kroger spokeswoman Rebecca King asked, then answered her own question.

KPRC Local 2 gave Kroger a copy of every receipt with an overcharge so the store could do its own research.

"We feel that a good way, an accurate way to base this study is based on the number of items that you actually purchased," King said.  "So the researcher purchased about 1,057 items specifically. We felt that there was a potential for error on about 26 of those items, which is an error rate of about 2 percent."

King said in some cases she thinks Davis may have picked up the wrong item or perhaps a sale sign stayed up long after a sale was over.

"In some cases, it's just really hard to tell," King said.

Still, at the end of the year, instead of getting short-changed, Davis came out ahead, and so did the Houston Food Bank.

"We found $145 that we wanted to donate to the Houston Food Bank," Davis told food bank president Brian Greene.

"Oh, well, thank you very much," said Greene. "How did you find the money?"

Good question. Kroger's "scan right guarantee" promises that if any item you purchase scans incorrectly at the register, you get one of that item for free, up to $25.

For example, Tropicana orange juice was on sale for $2.50. At the register, Davis was charged $2.88. According to Kroger's "scan right guarantee," she gets a refund of not just the 38 cents she was overcharged, but the full price of the product, $2.88.

Our $60.99 in overcharges turned into a refund of $142.61. We rounded up and gave the Houston Food Bank $145.

"This money is very much appreciated," said Greene. "We're going to get a great multiplier out of it and a lot of food is going to go out."

When we told you about "Operation Price Check" in July, a lot of KPRC Local 2 viewers e-mailed Davis and said they have a hard time getting the cashier to honor the store policy and give them a full refund on the entire price of the product.

There is a method.  First, we said Davis did all of her shopping for "Operation Price Check" at Kroger, but Randalls also has the same policy that if any item scans incorrectly at the register, the customer gets one of that item for free. The cashiers won't always tell the customer about the refund. Here's how to make sure you get one.

We all know no one likes the customer who holds up the line haggling over 20 cents. Davis has gotten some pretty dirty looks herself. Instead of watching the prices like a hawk at the checkout, take some paper and a pen with you when you shop and write down the price of every item you put in your basket.

After you pay, take your receipt and wheel your cart out of the way, but not out of the store.

You want to make sure the prices on your receipt match the prices you marked on your list. If you find any errors, circle them. Then take the receipt and your purchases to the customer service booth and tell them you want a refund on those items because they scanned incorrectly.

"It sounds like a good policy," said Lee. "They've always been good about changing it at the register. I just wasn't aware you could actually get a free item."

"There are between 70,000 and 120,000 items in every store and with thousands of price changes each week, unintentional errors may occur," said King. "And when you find those, please bring those to our attention and you'll be compensated for them."

Kroger's policy only applies to items without a price tag on the package. The store will not give you a refund for alcohol or tobacco.  Randalls' policy applies to everything except alcohol, tobacco and dairy products. We checked with both H-E-B and Walmart, but neither of those stores has a price guarantee policy.