You can send us a CSV file of each SKU's stock level you want us to update in bulk. These can also be offsets but you'll have to let us know beforehand that's how you would like things adjusted.
During the import:
If the SKU doesn't exist, it will not be updated
SKUs are not case sensitive–
ReD-BriCkwill still match
Once you have your file ready, you will need send it over to us via support. Unfortunately there's no way to upload a CSV yourself at the moment but we hope to introduce a self-serve option in the future.