Inventory management for my small ecommerce site (wordpress), website is not live yet

I run a small-scale ecommerce site and keep every SKU’s quantity inside the website’s own admin panel. What I am missing is an automatic way to keep those numbers in sync with real-time sales activity.

Here is what I need built and fully tested:
• Whenever a product is purchased, the system must instantly deduct the sold units from the on-hand quantity.
• As soon as a product’s count reaches zero, it should switch to “Out of Stock” status on the storefront so customers can’t buy what I don’t have.

All updates come from— and write back to— the same internal inventory database; no supplier feeds, POS integrations, or external spreadsheets are involved. The final solution should slot neatly into my existing admin workflow, require minimal manual upkeep, and include clear setup notes so I can maintain or adjust it later.

If you have experience developing inventory automation for ecommerce platforms (custom PHP, WooCommerce, Shopify apps, or similar), I’d love to see a quick outline of your approach and a rough delivery timeline.