Developed robust Fulfillment Inbound API validation and credential interfaces for the amzn/selling-partner-api-samples repository, focusing on enhancing reliability and security for partner integrations. Leveraged Java and AWS to implement comprehensive input validation across fulfillment inbound API operations, consolidating validation scaffolding to ensure consistent data contracts. Introduced enum classes to define API response keys, streamlining response handling and improving maintainability. Designed an interface for secure credential provisioning, laying the groundwork for scalable credential management in backend systems. The work addressed critical integration requirements, emphasizing secure API interactions and preparing the codebase for future expansion while maintaining a clear, maintainable structure using Java and API integration best practices.