Unfortunately, the MWS API (the system Amazon allows 3rd parties to communicate with them through) has not been updated to handle combining commingled inventory with seller labeled inventory. For more information on this topic please see- What are the Amazon MWS Limitations?
The good news is, to bypass this limitation, you can create and upload an inbound shipping plan to Seller Central for the items listed through Stratify by following the steps below!
To begin, you will need to add the items to a Private batch.
After everything that needs to be shipped has been added, click the Review Batch button at the bottom of the List & Prep page.
On the Review Batch page, click Submit. This will bring up a window that gives you an option to send your product listing feeds, or send feeds and request proposed FBA shipment plans.
When the window appears, uncheck the box for the Request proposed FBA shipment plans from Amazon option, then click Sync. This will close your batch and allow you to begin the process of uploading the shipping plan into Seller Central.
From here, go to List>Closed Batches:
Click the name of the batch you need a Shipping Plan File for:
Click Export and choose Send to Amazon Step 1 from the dropdown menu. This will generate a properly formatted .txt file that can be imported to Seller Central's Send to Amazon page.
When the prompt appears, you can change the name of the file before clicking OK to save it on your computer.
Now, you're ready to upload! Navigate to Seller Central>Inventory>Shipments>Send to Amazon or visit this link (click here) to get started.
- Please note that you must wait for 15 to 30 minutes for your feeds to fully sync your listings over to Amazon prior to uploading your Shipping Plan file into Seller Central. If you attempt the upload too early, Amazon will give an "MSKU not found for one or more SKUs in the destination marketplace" error.
For help with uploading the file to Seller Central, please reference the remaining steps here- Uploading a Shipping Plan File using Send to Amazon