Amazon charge their prime users a membership fee of $99 in the United States and although the fee is $20 up from its initial rate introduced in 2015 the company managed to double the number of Prime members within the last couple of years.

The service started by offering free two-day shipping as well as discounted one-day shipping rates. The service gradually evolved to provide free same-day delivery, unlimited video streaming, free access to a large music library, photo storage and other benefits.

A report by the Consumer Intelligence Research Partners (CIRP) says Amazon Prime now has 80 million subscribers in the US, which is double the customer base of 40 million users in March 2015. A year earlier the number of Amazon Prime members stood at 58 million.

Amazon is profiting not only from the growing membership base but is creating loyal customers that tend to spend more, the report finds. The average Prime member spends $1,300 a year on Amazon while a non-member spends half of this amount, $700 a year. Thus, Amazon Prime program generates some $104 billion in sales annually, a hefty figure that few other online retailers can achieve.