Submit form to create estimate

Hi

I'm really hoping someone can help me understand the deluge script that is needed to make this work! Basically I have a form which includes a sub form in it.

Between the forms they collect customer data, such as name, address, contact number and information about rooms in a house such as measurements and items that might be installed in them, all combined they will be used to provide a quote/estimate.

The form is all working nicely, however, I now need to add some script to send some elements of the form across to Zoho Invoice as an estimate.

I have seen the examples put up in the knowledge center (https://help.creator.zoho.com/Create-Estimate.html) but don't fully understand it, for example, in the script is says to include the Customer ID (

refers to the name of the expense category. ( Mandatory) - whats an expense category???...

customerID = 141995000000022001;
send = true;
itemList = List:Map();
itemList.add({ "ItemName" : "Test", "ItemDescription" : "Sample Description", "Price" : 50 });
itemList.add({ "ItemName" : "Test1", "ItemDescription" : "Sample Description", "Price" : 50 });
estimateInfo = { "EstimateDate" : zoho.currentdate, "ReferenceNumber" : 123, "Custom.Body" : "This is the mail content", "Custom.Subject" : "This is mail subject", 
"Description" : "Comment added to an invoice", "Notes" : "Estimate Notes will be shown in PDF", "Terms" : "Terms and Conditions apply", "Items" : itemList };
resp = zoho.invoice.createEstimate(customerID, estimateInfo , send);

How do I find this customer ID? I want the customer id to be auto generated based on 
whatever the customer info has been completed on the form, so this ID doesn't exist as 
far as I know?

Does anyone have a working example where multi items are sent from Zoho Creator to an estimate or can someone help to explain what this customerID part is all about?

Thanks in advance for any help.