WooCommerce inventory should be based on Available Stock, not Stock on Hand

Now that Square tracks Available Stock and Stock on Hand as separate values, the plugin should really be using the Available Stock amount as the value synced with WooCommerce to prevent unavailable inventory from being double-sold through WooCommerce.

I have some products I’ve pre-sold via invoice which are technically no longer in stock; they won’t be removed from Stock on Hand until the invoice is marked “paid,” but they are still visible as available for sale through my website because the plugin is using the “Stock on Hand” value.