It's no secret that shoppers have been flocking to (NASDAQ:AMZN), and an increasing number of those customers have been signing up for Amazon Prime, the company's customer loyalty program. In exchange for an annual fee of $119 (or a monthly rate of $12.99), members are treated to a host of benefits that seems to grow with each passing year, benefits that are now available in 17 countries.

As 2018 draws to a close, let's review some of the more popular benefits that made Prime a worthwhile investment for shoppers this year.

A banner image that says "Prime Insider: 2018 Best of Prime," with caricatures of single and family customers.

Image source: Amazon.

1. Free shipping

Let's just get this one out of the way now, as the draw of free two-day or faster shipping is the crown jewel of Prime's offerings -- and likely the single biggest reason customers subscribe to Prime. This perk is available on more than 100 million items found on Amazon's website. 

That's just the tip of the iceberg, however, as Amazon has a growing list of items that qualify for free same-day delivery, free 2-hour delivery via Prime Now, and release-date delivery on pre-ordered items like Blu-rays and video games.

2. Prime Video

In addition to the thousands of licensed movies and television shows on Prime Video, Amazon has been investing in original content, spending an estimated $6 billion on programming in 2018. Some of the company's big bets have been paying off. The Marvelous Mrs. Maisel, for example, is the story of a 1950s housewife whose divorce leads to a life as a stand-up comedian. It was one of the most watched programs on Amazon Prime Video this year, and was a big winner at the awards shows as well, taking home a boatload of Golden Globes and Critics' Choice awards. Most notably, The Marvelous Mrs. Maisel won eight Emmys, the most ever for a comedy show during a single season.

Other member favorites included Tom Clancy's Jack Ryan, starring John Krasinski, and Homecoming, starring Julia Roberts, which became the two most binged shows on Prime Video. 

As an added bonus, members have access to Amazon Channels, which lets them subscribe to HBO, Showtime, Starz, and a host of other popular cable and streaming channels for a monthly fee of $4.99 to $14.99 per channel -- all without a cable subscription or commitment -- which they can cancel anytime. 

An Amazon box with variety of animated items and locations and the caption Prime Day starts July 16, 3PM ET.

Image source: Amazon.

3. Prime Day

Amazon's annual Prime Day sale has become a member favorite, and the company's biggest shopping event of the year. In 2018, Amazon expanded the sale from 30 hours last year to 36 hours this year, offering Prime members more than 1 million items at deeply discounted prices. New subscribers flocked to buy Prime memberships -- the company "welcomed more new Prime members on July 16 than any previous day in Amazon history." 

4. Whole Foods discounts

Amazon acquired Whole Foods last year, and Prime members can now save on exclusive deals on select items throughout the store, and get an additional 10% off items that are on sale. The company also offered additional exclusive Whole Foods deals to members during its recent Prime Day sale.

If that weren't enough, customers with the Amazon Prime Rewards Visa get 5% cash back on all purchases at both Whole Foods and, as well as 2% cash back at restaurants, gas stations, and drug stores, and 1% back on all other purchases. Prime members also benefit from free 2-hour delivery of Whole Foods groceries through Prime Now (where available).

5. Prime Music

Amazon hasn't forgotten the music lover, offering unlimited ad-free access to hundreds of Prime playlists and more than two million songs on Prime Music to members in the U.S. and Puerto Rico. Prime members also get exclusive discounts to Amazon Music Unlimited, which offers both monthly and annual subscription plans.

The company revealed that the most streamed musical artist was Justin Timberlake, and favorite albums included Man of the Woods by Justin Timberlake, Camila by Camila Cabello, Astroworld by Travis Scott, and Mitchell Tenpenny EP by Mitchell Tenpenny.

A young man sitting on the bed in the back of a van reading a Kindle.

Image source: Amazon.

6. Prime Reading

Another perk is unlimited access to ebooks and books with audible narration via Kindle devices, Fire Tablets, or other devices using the Kindle App on iOS or Android.

The five most borrowed titles by U.S. Prime members were Harry Potter and the Sorcerer's Stone, The 7 Habits of Highly Effective People, The Handmaid's Tale, Milk and Honey, and Say You're Sorry. The most beloved comic-to-movie adaptation was Black Panther.

More to come

These are just six of the many benefits available to Prime members. This is a win-win for subscribers, as well as for Amazon. Prime members tend to be among the most lucrative for the company, spending more than twice the amount of the average customer according to research by Consumer Intelligence research Partners. Another telling metric is that in the U.S., 61% of Amazon customers are Prime members according to the research. 

It's difficult to imagine what Amazon will add next from Prime members, but given the company's track record, customers will love it.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.