Integrated WordPress Retail POS

I need a custom point-of-sale solution that keeps my retail store, WordPress site, and countertop POS terminal working as one. The goal is to enter a product or complete a sale in any channel—online or in-store—and have inventory levels update everywhere within seconds... (Budget: $30 - $250 AUD, Jobs: API Integration, Inventory Management, MySQL, PHP, Point of Sale, Software Architecture, WooCommerce, WordPress)