Shopify lets a variant keep selling after its stock reaches zero, and its stock can drop below zero. Etsy cannot sell anything with no stock. The Out of stock and negative stock section tells Shuttle what to send to Etsy in these cases. You can keep backorder items buyable on Etsy, or stop negative Shopify stock from changing your Etsy listings.

Both settings are in Settings > Stock Syncer > Shopify → Etsy, in the Out of stock and negative stock section.

To open the Stock Syncer settings, you need the Growth plan or higher, or an active Stock Syncer trial.

Keep variants that continue selling in stock on Etsy keeps a variant buyable on Etsy while it keeps selling on Shopify. When the setting is off, Shuttle sends 0 to Etsy once that variant runs out, so Etsy shows it as sold out.

The setting applies to a variant when all of these are true:

Once you add stock to the variant in Shopify, Shuttle syncs its real quantity again.

The setting applies to stock syncing from Shopify to Etsy. When stock syncing from Shopify to Etsy is turned off, Shuttle sends no stock to Etsy, so the setting only stops stock syncing from Etsy to Shopify from overwriting these variants.

Shuttle sends your Default stock quantity instead of 0. If Default stock quantity is empty, Shuttle sends 1.

You set Default stock quantity in Settings > Products > Shopify → Etsy, as a whole number from 1 to 999.

The quantity Etsy shows for these variants keeps them buyable. It is not real stock.

Some Etsy listings have one quantity for the whole listing, or one quantity for each value of an option, such as one per size. For these listings, Shuttle uses the lowest Shopify quantity among the variants that share that quantity.

When Keep variants that continue selling in stock on Etsy is on, a variant that keeps selling at 0 or below does not pull the shared quantity down to 0. Shuttle uses the lowest quantity of the other variants instead. For example, if a Small with 4 in stock and a Large that keeps selling at 0 share one quantity, Etsy gets 4.

When every variant that shares the quantity keeps selling at 0 or below, Shuttle sends your Default stock quantity, or 1 if it is empty.

See Handling Etsy Stock Management by Product or Option Level.

The Etsy quantity of a variant kept buyable this way is not real stock, so Shuttle never copies it back to Shopify. When stock syncs from Etsy to Shopify, Shuttle skips these variants, leaves their Shopify stock as it is, and records a warning.

See Shopify variant continues selling when out of stock, skipped.

Ignore negative stock levels decides what Shuttle sends to Etsy when a variant's Shopify stock is below zero:

On listings that share stock, Shuttle checks the shared quantity. When Ignore negative stock levels is on and that quantity is below zero, Shuttle leaves the Etsy quantity as it is.

Ignore negative stock levels does not apply to variants that Keep variants that continue selling in stock on Etsy covers. Those variants always get your Default stock quantity, or 1.

For negative stock going from Etsy to Shopify, see Handling Negative Stock When Syncing Quantities from Etsy to Shopify.

To use the Stock Syncer settings, you need the Growth plan or higher.