What data source should be used while a legacy billing system's SOAP endpoint is not ready for a FlexCard design?

For a FlexCard design where a legacy billing system's SOAP endpoint is not yet available, Integration Procedures are the most suitable data source. Integration Procedures are designed to orchestrate multiple calls and transform data seamlessly without the overhead of needing a direct connection to a SOAP endpoint.

They can aggregate data from various sources and allow you to handle more complex logic flow, which is particularly useful when you need to retrieve data in a way that aligns with the structure of your FlexCard. By utilizing Integration Procedures, you can decouple the data retrieval from the legacy system and manage the data within Salesforce itself until the SOAP endpoint is operational.

In contrast, DataRaptors serve as simple data fetching and transformation tools, but they are typically utilized when you have a straightforward requirement. Apex REST would require a custom solution to fetch data and isn't as streamlined for situations where the backend is not yet prepared. Apex Remote functions similarly to Apex REST but is more suited for specific scenarios involving Visualforce pages rather than FlexCard designs. Therefore, Integration Procedures offer the right balance of flexibility and functionality for handling data in this interim period.