Cost/Unit, Supplier, and Purchase Date can be updated individually on the replenishment detail pages for an MSKU. Just follow the steps below!
1. Click on Inventory at the top of your screen and select either "FBA" or "Merchant", depending on what fulfillment channel your item is in.
This will take you to the Main Inventory page
2. Search for the item you'd like to update by using keywords, Title, ASIN, MSKU, tag, or scanning the UPC in search box showing in the upper right corner.
3. Click on the item to be taken to the replenishment detail page.
4. Click the Cost/Unit, Supplier, or Purchase Date fields to add in your purchasing information.
If you have a lot of items to update, you can also import your buy cost in bulk using a .csv by following these instructions to save time!