Repeat customers may have noted an interesting phenomenon when purchasing DVDs at Amazon.com: fluctuating prices. The online retailing giant apparently has been performing random testing with customers, resulting in variable prices that affected more than half of the company’s top-100 best-selling DVDs. Depending on the particular movie and the discount Amazon offered, one customer could pay up to $15 more for a DVD than another customer.

Amazon spokeswoman Patty Smith said the variable prices are part of a pricing test going on in Amazon’s DVD department. Smith said the company routinely tests different aspects of its site, including price, to see how its customers will react.

Smith declined to say how long the current test had been going on, saying only that it is ongoing and that it will continue for only a ‘limited’ time. Amazon has not alerted customers about the price test, Smith said.

‘They could pay a higher price and not realize it, but they could also pay a lower price and not realize that they paid it,’ she said.

Flak over the pricing test on DVDs comes just four months after a similar pricing test by Amazon on a popular MP3 player. As with the DVDs, customers affected by the MP3 pricing test expressed anger at the company, saying they felt hoodwinked by the random prices.

Amazon customers reported that certain DVDs had three different prices, depending on which ‘cookie’ a customer received from Amazon. Cookies are small files that Web sites transfer to customers’ computer hard drives through the browsers they use. These files allow sites to recognize customers and to track their movements and purchases.

One DVD affected by the pricing test was the limited edition version of Men in Black, which is currently the third best-selling DVD on Amazon. The list price for ‘Men in Black’ is $39.95, but Amazon is offering the DVD for $23.97, $25.97 and $27.97 to different customers.

Meanwhile, the company is offering The X-Files: The Complete Second Season for $89.99, $97.49 and $104.99 to different customers. The list price for the DVD is $149.98.

Smith said customers who discover a lower price on an item they have already ordered can ask the company for a refund. But some customers have reported problems in getting customer service to acknowledge the variable pricing.

The bottom line: as long as you’re surfing the web anyway, check around for the best prices. Hey, depending on your cookies, you may end up with a terrific discount.