Those of us who were around for the early days of the internet remember when Amazon was just a bookseller, or "Earth's biggest bookstore," as the company fondly described itself.

Amazon has since expanded to just about every market imaginable, becoming one of Earth's biggest companies altogether. But books -- even the physical kind -- remain a popular category, and Amazon is a big platform for large and independent booksellers to distribute their titles.

Starting Aug. 31, those third-party merchants have to adapt to a significant and costly change. According to an email that Amazon sent to sellers of books, CDs and DVDs, distributors to the lower 48 states will have to deliver items within a window of four to eight days, down from four to 14 days.

"We are launching new capabilities to help you set faster delivery times for your Seller fulfilled products. Customers are more likely to purchase products that have a faster estimated delivery time at checkout," the company said in an email to sellers that was obtained by CNBC.

For Amazon customers, particularly Prime members, this type of change may seem antiquated. Amazon has trained us to expect everything to arrive in two days, and in some cases faster. The company is putting in place a number of policies that force sellers to provide the same speed and quality of service that customers have come to expect from Amazon. Last week, CNBC reported on a new measure that makes it easier for buyers to get refunds at the seller's expense.

But marketplace merchants, who sell out of their garage or warehouse rather than using Amazon's fulfillment centers, have to build shipping costs into their business model, and they typically operate on very thin margins.