Mastercard Inc. on Tuesday launched a buy now, pay later service, tapping into a market that is threatening to chip away at the dominance of credit cards as a main source of payments. The buy now, pay later sector got a boost during the COVID-19 pandemic as cash-strapped shoppers were attracted to the service due to the ease of making part-payments for products they bought online or at stores, without additional costs or fees. That, in turn, has led to a surge in business at some of the biggest players in the market, including Klarna, Affirm Holdings, Afterpay Ltd. and PayPal Holdings Inc., while driving a string of big takeover deals. In August, Square Inc.—the payments firm of Twitter co-founder Jack Dorsey—bought Afterpay in a $29 billion deal, while earlier in September, PayPal said it would acquire Japanese BNPL firm Paidy in a $2.7 billion deal. Mastercard said on Tuesday …