Amazon.com Inc. has launched a referral program for its Prime two-day shipping program. The referring online shopper receives a $5 Amazon credit after the new customer signs up for Prime and makes a purchase.

The program, promoted today on the Amazon.com home page, applies to all Amazon customers, not just those who belong to Prime, a spokeswoman for the web-only retailer says. Prime members typically pay $99 per year to receive free two-day shipping on eligible products plus other benefits such as free streamed content, though Amazon offers free memberships to students and other consumers.

“Through e-mail and Facebook, sponsors can easily invite their friends to learn about the program and signup for a 30-day free trial,” she says. “There is no limit on the number of friends a sponsor can refer.”

The Prime referral program marks at least the second time in recent months that Amazon has offered an incentive to members of its shipping program. In late July, Amazon launched a “no-rush” shipping option for Prime members. In return for a bit of patience—instead of getting their goods within two days, customers will receive orders in five to seven business days—Amazon will reward those customers with a $1 credit that can be applied to titles offered by Amazon Instant Video.

Amazon today provided no update on how many consumers belong to Prime. Scot Wingo, CEO of ChannelAdvisor Corp., which facilitates retailer sales through Amazon and other web marketplaces, has estimated Prime membership at some 23 million and said those shoppers spend, on average, four times more than other consumers who shop at Amazon.com. That means Prime could reach some 50 million consumers given that a Prime customer can let family members use the service and taking into account free memberships Amazon has awarded to students and other groups.

Amazon is No. 1 in the Internet Retailer Top 500 Guide.