Custom E-Commerce WordPress Plugin

I need a bespoke WordPress plugin that adds full-blown e-commerce capability to an existing site without relying on WooCommerce or other third-party suites. The focus is on a lightweight, secure solution that slots neatly into the current theme and admin workflow.

Core features required
• Payment gateway integration
• Shopping cart

The checkout flow must support multiple currencies, allow me to toggle tax and shipping rules from the back-end, and run smoothly on both desktop and mobile. Clean, well-commented PHP, adherence to WordPress coding standards, and use of hooks/filters for easy future expansion are essential. Please plan for a settings page inside wp-admin where I can manage keys, adjust cart behaviour, and view basic transaction logs.

Deliverables
1. Installable plugin ZIP with source code
2. SQL (if any) for custom tables
3. Brief setup & user guide
4. Short video or screenshots proving successful test transactions in a sandbox environment

I’ll run final acceptance after end-to-end testing on a staging server, so your hand-off should include any dependencies and step-by-step instructions. Looking forward to your ideas on preferred payment APIs, security best practices, and UI polish.