A common error you can run into while working with Listing Mirror and Shopify is:
What this error means is that the Shopify listing has been edited in the Shopify marketplace directly. Doing so can cause a disconnect between the listing in Shopify and the listing in Listing Mirror and we won't be syncing qty or any changes made.
All listings (except for qualifying Amazon listings) should be edited directly in Listing Mirror. Once you've edited them and saved your changes, the changes will be synced out to the marketplace.
If you do edit a listing directly in your marketplace, the solution is to run the function delete and reimport function for your Shopify listing.
That function be found here.