Shopify Payment Integration & Page Tweaks

I need the new payment gateway app installed, configured, and fully tested so customers can complete a purchase without friction and so transactions reconcile correctly on the back end. Alongside that integration work, I want a light but precise refresh of my product pages and the checkout page... (Budget: ₹12500 - ₹37500 INR, Jobs: API Integration, CSS, HTML, JavaScript, Payment Gateway Integration, PHP, Shopify Development, Website Design)