Shopify Inc. /zigman2/quotes/209033712/composite SHOP +2.84% on Wednesday said it has selected Affirm, a buy-now-pay-later (BNPL) financing service for credit-card users, to exclusively power Shop Pay Installments when it launches later this year. Terms were not disclosed. Affirm, founded by PayPal Holdings Inc. /zigman2/quotes/208054269/composite PYPL +2.49% co-founder Max Levchin, has seen a 40% increase in merchant signings from March 1 to April 30, compared to the first two months of the year. Among the 6,000 merchants who use it are Walmart Inc. /zigman2/quotes/207374728/composite WMT +0.90% and Peloton Interactive Inc. /zigman2/quotes/208035743/composite PTON -0.43% . "This is one giant leap to make our service ubiquitous," Levchin told MarketWatch in a phone interview Wednesday. "This has changed the game. The BNPL market is on a tear."