Ecommerce Product Page Fixes

My ecommerce store runs on a custom PHP + HTML stack and the product pages have a few broken elements I need put right. The core template is stable, but several features that used to work flawlessly are now patchy:

• Sale price badge no longer appears when a product is discounted
• Variant selector sometimes skips options or won’t trigger the AJAX price refresh
• Thumbnail carousel occasionally loads out of order, especially on mobile

All the logic is already there, it just needs debugging and clean, maintainable corrections—not a complete redesign. I will grant Git/SFTP access on a staging copy so you can test freely before pushing to production.

Please show me a comparable fix or small module you have repaired in the past so I can gauge your approach and code style. If you spot any quick-win improvements as you work, let me know; I’m happy to expand the scope in separate milestones once the essential bugs above are resolved.

Deliverable: updated PHP and HTML/JS files (or override snippets) that restore full, cross-browser functionality on every product page, together with a short change log so I can track what was modified.