Amazon.com plans to offer brick-and-mortar retailers a checkout system that uses Kindle tablets as soon as this summer, people briefed on the company’s plans said. In one scenario, the Seattle company would give merchants Kindle tablets and credit-card readers, the people said. Amazon also might offer retailers other services, such as website development and data analysis, the people said. To accelerate the project, Amazon last year bought certain technology and hired some engineers from GoPago Inc., a San Francisco startup that offered checkout systems linked to a smartphone app, the people said. GoPago is now a unit of DoubleBeam Inc. Amazon’s plans remain fluid and the project might be delayed, altered or canceled, they said. The project would thrust Amazon, the largest U.S. e-commerce merchant, into the realm of physical retail stores, where more than 90% of commerce is still conducted, and open up a new trove of data from consumers’ in-store spending habits.