Some Etsy listings keep one quantity for the whole listing, or one quantity per option such as Size, while Shopify always counts stock for each variation. So when a Shopify sale updates one of these Etsy listings, the two shops can end up showing different numbers. The two mirror settings let Shuttle copy the result back to Shopify, so every related Shopify variation matches Etsy again.

Shuttle mirrors updates back to Shopify only after a stock change on Shopify has updated an Etsy listing. This happens when the Etsy listing:

Shuttle does not mirror listings with only one variation, because Etsy and Shopify already match there. Mirroring also never starts from a change made on Etsy. It only follows an update that Shuttle made to Etsy.

Mirror quantity updates to Shopify sets each Shopify variation to the quantity its matching Etsy variation has after the update. It is on by default.

For example, say Etsy tracks stock on Size and someone buys a Small Blue shirt on Shopify. Shuttle updates the Small quantity on Etsy. With Mirror quantity updates to Shopify on, Shuttle then gives the other Small variations in Shopify, such as Small Red and Small Green, the same quantity.

When Etsy keeps one quantity for the whole listing, Shuttle sets that quantity to the lowest quantity among the Shopify variations. Mirror quantity updates to Shopify then gives every Shopify variation that same number.

See Avoid Overselling on Etsy by Assigning Lowest Shopify Variant Quantity.

Mirror SKU updates to Shopify copies a SKU that Shuttle changed on Etsy back to every related Shopify variation. It is off by default.

This matters when Etsy keeps one SKU for the whole listing, or one SKU per option. In that case Shuttle gives Etsy the first Shopify SKU it finds for that listing or option. With Mirror SKU updates to Shopify on, Shuttle then writes that same SKU onto every Shopify variation that belongs to that listing or option.

Mirror SKU updates to Shopify only works when Sync product SKU changes between platforms is also on. That setting is under Settings > Stock Syncer > General, in the Sync product SKUs section.

Sometimes Etsy will not take new SKUs on a listing whose variations share their stock. Then the SKUs on Etsy stay as they were, and Shuttle does not copy any SKU to Shopify, whichever mirror settings are on.

See SKUs were left as they are because this listing shares its stock between variations.

Turn the mirror settings on if your Shopify variations should always show exactly the same quantities and SKUs as Etsy.

Leave them off if your Shopify variations need different quantities or different SKUs from each other. For example, you might count stock for each color in Shopify but only for each size on Etsy. Mirroring would overwrite those separate Shopify values with the Etsy value.

The mirror settings appear when Update the Etsy product quantity from Shopify variations or Update Etsy variations based on the option that manages stock is ticked. Both are ticked by default.

If you untick both settings in step 3, Shuttle stops updating stock on Etsy listings that keep their quantity on the listing or on an option, and the mirror settings are hidden. Hiding the mirror settings does not turn them off. They keep their saved values, so Shuttle still mirrors when an Etsy SKU is shared by several Shopify variations, or when Shuttle changes SKUs on Etsy. To stop all mirroring, untick both mirror settings before you untick the settings in step 3.

To use the Shopify → Etsy stock settings, you need the Growth plan or higher.

See Handling Etsy Stock Management by Product or Option Level.