When several variations stand for the same physical item, they should always show the same stock. The Update all duplicate SKUs option takes care of this: a quantity change on one SKU is copied to every other variation with the same SKU, on both Etsy and Shopify.
The products don't have to be matched to each other. Update all duplicate SKUs works whether or not they are matched.
Turn on stock syncing for all duplicate SKUs
In Shuttle, open Settings.
Under Products, choose General.
Find Update quantity for duplicate SKUs.
Tick Update all duplicate SKUs and save.
The Stock Syncer settings show the same option with the same label. Go to Settings, then Stock Syncer, then General, and look in the Sync stock of duplicate SKUs card for Update all duplicate SKUs. Both checkboxes are one setting, so turning it on in one place turns it on in the other.
To use the Stock Syncer settings, you need the Growth plan or higher.
Line up duplicate SKUs that already show different stock
Update all duplicate SKUs only affects quantity changes that happen after the option is turned on. That is true whether you turn it on in the Products settings or in the Stock Syncer settings.
Saving the option does not start a sync. Shuttle does not replay earlier Etsy quantity changes or sales, and it does not bring variations that already show different stock into line.
To line up existing duplicate SKUs straight away, set the quantity once in Quick Stock Updates:
Make sure Update all duplicate SKUs is on.
In Shuttle, open Inventory and go to Quick Stock Updates.
Search for the SKU.
Enter the quantity in the Etsy column and click the Update button next to it.
Shuttle sends the Etsy quantity you enter to every variation with the same SKU, even if the Etsy quantity itself has not changed.
Sync duplicate SKUs only within the updated product
If you want duplicate SKUs synced only within the same product and its match, use Limit duplicate SKU syncing to the updated product instead of Update all duplicate SKUs.
Only one of the two options can be on at a time. Turning on Update all duplicate SKUs turns off Limit duplicate SKU syncing to the updated product.
Example: one Etsy listing, separate Shopify products and a kit
Say you have an Etsy listing with three variations, with the SKUs AAA, BBB and CCC.
On Shopify, those aren't three variants of one product. They are three separate products, one for each SKU: AAA, BBB and CCC.
You also sell a kit on Etsy, and one of the kit's variations has the SKU BBB too.
With Update all duplicate SKUs turned on, a sale of the BBB variation on Etsy means Shuttle also updates:
The quantity of the Shopify product with the SKU BBB
The quantity of the kit variation on Etsy with the SKU BBB
How many products share one SKU's stock update?
When Update all duplicate SKUs is on, Shuttle updates up to 10 products on each platform for each SKU.