BigCommerce REST Order Integration

I need a lightweight REST API link between my order-management app and BigCommerce’s inventory service. The goal is simple: pull new orders, update stock levels, and push basic status changes back to BigCommerce.

Here’s what I’m after:
• A small, well-commented script or endpoint map that authenticates with BigCommerce, fetches orders, and updates inventory counts.
• Setup instructions (API keys, callback URLs, sample cURL calls) so I can run or schedule it myself.
• Basic error handling and clear JSON examples for each request/response.

Keep the solution straightforward—no fancy dashboards or advanced reporting, just a clean proof-of-concept that shows the data moving correctly.