You'll see an Overdue balance if you click on Vendors and see the Amazon vendor. The QuickBooks Online API does not allow a Bill payment to link a deposit to a vendor credit, so this needs to be done manually.
You'll need to click on Expenses and then on the Vendors tab. Select "Make payment" to create a bill payment. Select the Amazon Undeposited Funds account for the Bank/Credit account. Make sure all deposits and respective vendor credits are selected. See below for a visual guide.
Verify that the balance for the bill payment is $0.00 as all deposits should reconcile with the vendor credits. If they do not, check SellerZen make sure that all transactions are imported into QuickBooks.
Once the bill payment is saved, the overdue amount will be reduced by the amount in the deposits in the bill payment. In most cases the overdue amount will be zero.