I started by outlining the high-level flow of the checkout process, from cart review to payment confirmation. I emphasized the importance of a smooth user experience, highlighting features like guest checkout, promo code application, and real-time inventory updates. The interviewer seemed engaged but pushed back on my choice of a monolithic architecture, noting scalability concerns for high-traffic sales events. My response focused on microservices for modularity, but I could have provided more details on potential bottlenecks.