An investigation by ProPublica has shown that Amazon’s search algorithms direct customers to its own products or those from companies that pay for its Fulfilled by Amazon program, rather than the cheapest items.

The report checked 250 popular products over the course of several weeks to see which products were given prominence in the so-called “buy box,” which appears first as a suggested purchase. It was found that items sold by Amazon, or companies that pay the retail giant to ship products on their behalf, appeared in the buy box “about three-quarters” of the time – even when they weren’t the best deals.

ProPublica found that the average price difference between the products that Amazon recommended and the cheapest available on the site was $7.88. A customer who bought all 250 items from the buy box, rather than purchasing the lowest priced goods from other vendors, would spend 20 percent more – or about $1400 extra.

It was also found that shipping costs for Amazon-friendly goods are omitted when customers compare prices of items using the “price+shipping” search. While this may be correct for $99 per year Prime members and those purchasing more than $49 worth of items, it’s doesn’t paint an accurate picture for everyone else.

Responding to the report, Amazon said: "With Prime and Super Saver Shipping (which requires no membership and ships orders above $49 for free), the vast majority of our items ordered – 9 out of 10 – can ship for free. The sorting algorithms the article refers to are designed for that 90% of items ordered, where shipping costs do not apply."

The company went on to say that a number of factors are taken into account by its algorithms when determining which products appears in the buy box. These include seller rating, closest item to the customer, price, and free delivery, along with other components that it did not reveal.

Customers trust Amazon to have great prices, but that's not all— vast selection and fast, free delivery are also critically important. These components, and more, determine our product listings to ensure that customers have the best overall experience when shopping on Amazon. If a customer is solely looking for the lowest price available, as you have with this study, we clearly alert them on the product detail page that lower prices may be available from other sellers.