(Bloomberg) -- Amazon.com Inc. will allow eligible US retailers using the online retail giant’s Amazon Pay service to offer Affirm Holdings Inc.’s buy now, pay later product under a new agreement between the two companies.

The service, known as Adaptive Checkout, offers biweekly and monthly payment options for purchases of more than $50 with annual percentage rates starting at zero, according to a statement Wednesday. Millions of customers using Amazon.com and the Amazon mobile app already have access to the pay-over-time service, the companies said.

The buy now, pay later option can increase sales and customer loyalty, and attract new customers, San Francisco-based Affirm said. The mattress-and-bedding company Casper and water-filter supplier USA Berkey Filters have already integrated Affirm’s offering within their use of Amazon Pay, according to the statement.

“Customers want more choice and flexibility when paying online,” Affirm President Libor Michalek said in the statement.

Affirm shares jumped as much as 23% Wednesday morning, to $19.50, their biggest intraday gain since early February.

--With assistance from Bre Bradham.

(Updates with shares in last paragraph.)

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