DOS Letter and NVC Invoice ID

[arangoon]

We never received a letter from NVC on the status of my Parents GC and all of a sudden Received a letter from DOS stating visa is available and we need to login to CEAC to complete the application. However, one of the required fields is "Invoice ID" which I am not sure what is.

What is the next step is for us to complete the application?
Thanks

[arangoon]

I have the NVC case number which means our case is approved and we should have received a letter or an email from NVC case number and a separate invoice ID number. which we never did.

How do I obtain the Invoice ID?