Quadru has been discontinued.
Quadru compared and synced a Square Item Library with a Shopify catalog — products, inventory, and SKUs across both platforms.
Shopify no longer accepts apps that connect to a point-of-sale system outside of Shopify. Quadru is such an app, so it has been withdrawn from the Shopify App Store and is no longer operated as a hosted service.
If you used Quadru: no action is needed. The app had no paying merchants at the time it was withdrawn. You can remove it from your store under Settings → Apps and sales channels.
If you want to run it yourself: the source is published under AGPL-3.0. You can host it on your own infrastructure and install it on your store as your own Shopify app.
Quadru · AGPL-3.0 · “Square” and “Shopify” are trademarks of their respective owners.