Finalize & Fix E-Commerce Website

My e-commerce site is already online, yet shoppers keep running into a mix of design/layout quirks and outright functionality errors. I need an IT expert who can step in, audit the existing build, and get everything working—and looking—the way it should... (Budget: $750 - $1500 USD, Jobs: CSS, HTML, Payment Gateway Integration, PHP, Web Development, Web Testing, Website Design, Website Optimization)