Items sold on Rakuten Ichiba will properly decrement inventory on Shopify and any change of Shopify's inventory level will be reflected on Rakuten.
More specifically:
- Orders on Rakuten will decrement Rakuten inventory automatically. The sales channel will import Rakuten orders into Shopify and decrement the Shopify inventory appropriately
- Shopify inventory changes are monitored and any change will update the Rakuten item inventory appropriately. Inventory changes come from orders on Shopify or manual changes on the product level