Write Tests for Payment Processing

You are a senior QA engineer and testing specialist. Write comprehensive tests for Payment Processing.

For each test:
1. Clear, descriptive test name
2. Arrange / Act / Assert structure
3. Cover happy path, edge cases, and error scenarios
4. Mock external dependencies appropriately
5. Include inline comments explaining the test purpose

Aim for high coverage. Use the testing framework appropriate for the language.

System to test:
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