When the quote is approved in Creator, the invoice should already exist in Books. This function finds the customer (creating them if this is their first order), turns a Creator subform into invoice line items, and raises the invoice — writing the invoice number back so the two records stay linked.

Before you start

• A Zoho OAuth connection named zoho_books_connection with ZohoBooks.contacts.CREATE, ZohoBooks.contacts.READ and ZohoBooks.invoices.CREATE.
• Your Books organization ID — it is required on every single call.
• Creator forms: Invoice (with an Item_Detail subform), Contact with a Zoho_Books_Contact_ID field, and Item with Zoho_Books_Item_ID.

Deluge function

// ============================================
// CREATOR -> ZOHO BOOKS / INVOICE
// Find or create the contact, then raise the invoice.
// ============================================

void ZohoBooks.createInvoice(int recordId)
{
    ORG_ID = "YOUR_BOOKS_ORG_ID";
    CONN   = "zoho_books_connection";

    rec      = Invoice[ID == recordId];
    customer = Contact[ID == rec.Contact];

    // --- 1. Reuse the stored contact id, or create the contact ---
    contactId = customer.Zoho_Books_Contact_ID;
    if(contactId == null || contactId == "")
    {
        newContact = Map();
        newContact.put("contact_name",customer.Company_Name);
        newContact.put("contact_type","customer");
        newContact.put("email",customer.Email);

        createResp = zoho.books.createRecord("contacts",ORG_ID,newContact,CONN);
        contactBlock = createResp.get("contact");
        if(contactBlock == null)
        {
            info "Books contact creation failed: " + createResp.toString();
            return;
        }
        contactId = contactBlock.get("contact_id");
        customer.Zoho_Books_Contact_ID = contactId;
    }

    // --- 2. Build the line items from the Creator subform ---
    lineItems = List();
    for each row in rec.Item_Detail
    {
        itemRec = Item[ID == row.Item_Name];

        lineItem = Map();
        lineItem.put("item_id",itemRec.Zoho_Books_Item_ID);
        lineItem.put("name",itemRec.Item_Name);
        lineItem.put("rate",row.Rate);
        lineItem.put("quantity",row.Quantity);
        lineItems.add(lineItem);
    }

    if(lineItems.size() == 0)
    {
        info "No line items on record " + recordId + " - nothing to invoice.";
        return;
    }

    // --- 3. Raise the invoice ---
    invoiceMap = Map();
    invoiceMap.put("customer_id",contactId);
    invoiceMap.put("line_items",lineItems);
    invoiceMap.put("date",zoho.currentdate.toString("yyyy-MM-dd"));
    invoiceMap.put("due_date",zoho.currentdate.addDay(30).toString("yyyy-MM-dd"));
    invoiceMap.put("reference_number","CR-" + recordId);

    invoiceResp = zoho.books.createRecord("invoices",ORG_ID,invoiceMap,CONN);
    invoiceBlock = invoiceResp.get("invoice");

    if(invoiceBlock != null)
    {
        rec.Books_Invoice_ID     = invoiceBlock.get("invoice_id");
        rec.Books_Invoice_Number = invoiceBlock.get("invoice_number");
        info "Invoice created: " + rec.Books_Invoice_Number;
    }
    else
    {
        info "Books invoice creation failed: " + invoiceResp.toString();
    }
}

Notes

• Store the contact id on your side. Searching Books for a customer on every invoice is a wasted call and a race condition waiting to happen — two quick submissions create two duplicate contacts. Save contact_id to the Creator record the first time and reuse it.
• Zoho Invoice is the same code, one word different. Swap zoho.books.createRecord for zoho.invoice.create and the scopes for ZohoInvoice.*. Note the Invoice task is create, not createRecord, and its connection argument is not supported from Creator — Creator uses the app’s built-in Zoho Invoice integration instead.
• Check the response block, not the string. Reading .get("invoice") and testing for null is more reliable than searching the raw response for a substring.
• Guard the empty subform. Books rejects an invoice with no line items with an error that does not obviously say so. Catching it in Creator saves the confusion.