Fix Credit Card Payment Issues

I'm looking for a skilled developer to fix an issue with our credit card payment system. Currently, transactions are failing at checkout, which is causing frustration for our customers and loss of sales.

Key requirements:
- Diagnose and resolve the issue causing transaction failures at checkout
- Ensure that the credit card payment method is fully functional and secure
- Test the payment system thoroughly before re-launch

Ideal skills and experience:
- Strong background in payment gateway integration
- Experience with troubleshooting payment processing issues
- Familiarity with security protocols for online transactions
- Ability to conduct thorough testing and provide a detailed report on fixes

Please provide examples of similar work done and your approach to ensure this issue is resolved efficiently.