An Integration Procedure is the most appropriate feature for this requirement. It is a declarative, server-side process designed to execute multiple actions in a single server call. It can orchestrate calls to different data sources, such as making an HTTP callout to an external system to fetch permit fees and simultaneously using a DataRaptor to retrieve discount information from a Salesforce custom object. It can then perform calculations on the aggregated data before returning the final fee amount. This aligns perfectly with the scenario's need for multi-source data retrieval and processing.

A. DataRaptor: A DataRaptor is used to retrieve, transform, or write Salesforce data. It cannot, by itself, make a callout to an external system to fetch data.

C. FlexCard: A FlexCard is a user interface component used to display contextual information and provide access to actions. It is not the tool used for the backend data orchestration and calculation logic.

D. OmniScript: An OmniScript provides a guided user path. While it can call an Integration Procedure or an HTTP action, its primary purpose is user interaction, not complex, server-side data aggregation from multiple systems. The Integration Procedure is the specialized tool for that backend logic.

1. Salesforce Help, OmniStudio Documentation, "Integration Procedures": "An Integration Procedure is a declarative, server-side process that executes multiple actions in a single server call. Integration Procedures can read and write data from Salesforce and from external systems (using REST API calls)... Use an Integration Procedure to access and transform data from third-party sources." This directly supports the use of an Integration Procedure to call an external system and process data.

2. Salesforce Help, OmniStudio Documentation, "DataRaptors": "DataRaptors are a configurable service for retrieving, transforming, and updating data." This documentation specifies their role is confined to data operations, primarily within Salesforce, and does not include orchestrating external API calls.

3. Salesforce Help, OmniStudio Documentation, "OmniStudio FlexCards": "FlexCards are cards that display contextual information and provide access to relevant tasks for a customer... They are the building blocks of the UI." This confirms FlexCards are a presentation-layer component.

4. Salesforce Trailhead, OmniStudio Architecture, "Choose the Right OmniStudio Tool": This module distinguishes the tools' purposes. It positions Integration Procedures for "bundling actions together to create a single, server-side service" and handling "data from multiple sources," while OmniScripts are for "creating a guided path for completing a business process." This architectural guidance confirms the Integration Procedure is the correct choice for the described data logic.