Web Developer Needed – Fix Checkout Issue + Site Testing & SEO

I’m looking for an experienced web developer to help resolve a few issues on my website and ensure everything is running smoothly.

Key Issue:
• When a user goes to purchase via the shopping cart, the form does not proceed correctly
• It refreshes/stays on the same page and highlights “apply coupon,” even though coupons are disabled
• The checkout process appears to think something is missing, but no clear error is shown

What I Need:
• Diagnose and fix the checkout/cart issue
• Ensure the purchase flow works seamlessly from start to finish
• Full website testing across desktop and mobile
• Identify and fix any bugs or broken elements
• Basic SEO check and optimisation (site speed, structure, indexing, etc.)

Bonus (if experienced):
• Suggestions to improve performance and user experience
• Advice on improving conversion rate

Looking for someone reliable who can start quickly and communicate clearly.