The postal system implemented Rural Free Delivery (RFD) in 1896. Since the country was literally wireless, telephone wireless, two-way communication was by post. RFD also made the mail order business possible. By permitting the classification of mail order publications as aids in the dissemination of knowledge, it entitled those catalogs a one cent per pound postage rate. That made the rural distribution of catalogues quite economical while the railroads provided distribution to delivery points.
The Sears, Roebuck and Co. catalog called itself the "Book of Bargains: A Money Saver for Everyone," and the "Cheapest Supply House on Earth," claiming that "Our trade reaches around the World." At the apex for mail order merchandise, you have the model website for its time that included testimonials from satisfied customers. The catalogue made every effort to assure the reader that Sears had the lowest prices and best values. The 1903 catalog included the commitment, "Your money back if you are not satisfied."
The point is that it is not just one thing that makes a milestone, but a combination of things that is transformative. The combination of catalogue, RFD, and the rail system made it easy for customers to buy.
Talk about making it easy, here are some more combinations for consideration. The increasing use of the credit card from 1958 is a significant development for consumers and culture. Add that to the introduction of the American Telephone & Telegraph (AT&T) 800 toll-free service in 1967, so that subscribers like Sears could allow their customers to reach them without toll charges, and you have a milestone.
The next milestone occurred when the development of an Internet from 1957 is coupled with the relative affordability of the personal computer from about 1986. Add to that combination the growth privately owned shipping services with incredible logistics like UPS and FedEx and by 1994 the Dotcom bubble is on with the founding of Amazon. The next year brought Craigslist, Yahoo and eBay. That being noted, the milestone is that consumers could look at an online catalogue, call a customer service agent, process and pay for an order and have it delivered the next day.