Integrate E-cart and Payment Gateway

I am looking for a freelancer who can integrate my PayPal payment gateway with my website's e-cart. The e-commerce platform used on my website is in-house, so the freelancer needs to have experience in integrating payment gateways with custom platforms. Additionally, I require the implementation of discount codes, Stock Management, and shipping for my e-cart. Ideal skills for this job include knowledge of PayPal API, PHP, and e-commerce platform integration.