Fix Checkout Functionality on Custom Site

I need a skilled developer to fix an issue on my custom HTML/CSS/JavaScript website. After recent updates to the user interface, added features, and improved functionalities, the checkout process now mimics the check-in process on the Google spreadsheet.

Requirements:
- Diagnose and correct the checkout process to ensure it operates independently from check-in.
- Ensure that all updates seamlessly integrate with the existing dashboard.

Ideal Skills:
- Strong proficiency in HTML, CSS, and JavaScript.
- Experience with custom web development and troubleshooting.
- Familiarity with Google Sheets integration and real-time data updating.

Please provide examples of similar work done.