If you don't want or need inventory tracking in QuickBooks, then you can use a non-inventory item for all of your SKUs.
Make a non-inventory item in QuickBooks Online for all of your Amazon inventory. Give it a descriptive name like Amazon US SKUs.
Then map that non-inventory item in SellerZen. We'll use this non-inventory item for all of your invoices.
Make sure to click on Save.