Explanation
Invoking middleware service to retrieve valid shipping methods is a solution that can allow the sales representatives to select between valid services for the customer's country. The middleware service can act as a single point of entry for all shipping services and provide routing and transformation capabilities. Using middleware to abstract the call to the specific shipping services is a solution that can allow the sales representatives to request shipping estimates from the selected service. The middleware can hide the complexity and heterogeneity of the shipping services and provide mediation and orchestration capabilities.
Using Platform Events to construct and publish shipper-specific events is not a solution, as Platform Events are used for event-driven integration, not for web-service integration. Storing shipping services in a picklist that is dependent on a country picklist is not a solution, as it does not address how to request shipping estimates from the shipping services. Reference: Salesforce Integration Architecture Designer Resource Guide, page 16-17