Write tests for payments

この issue には説明がありません。

The issue title mentions payments, but the body is empty. First, examine the repository structure to locate payment-related code, likely in a directory like `src/payments` or `lib/payments`. Look for existing test files to understand the testing framework used (e.g., Jest, Mocha). Determine what payment functionalities need coverage, such as transaction processing or error handling. Write tests based on the existing code's behavior and expected outcomes.

索引モデルが issue の本文から書いたものです。