You need to save customized items to Azure Cosmos DB.

1. API:

NoSQL

Reasoning: Since the development must use a native API that stores data in a document format, the NoSQL API is the correct choice. Azure Cosmos DB's NoSQL API stores data in JSON documents and is the most commonly used API for document-based use cases, which aligns with the storage of customized items in the scenario.

2. Consistency Level:

Session

Reasoning: The scenario requires that customized items maximize throughput while ensuring that the data is accurate for the current user. Session consistency provides the best balance between performance and data accuracy for scenarios where multiple reads and writes are performed by the same user session. It ensures that a user reads their own writes, which is important for ensuring that customized items are accurate for the current user.

Final Answer:

API: NoSQL

Consistency level: Session