Chasing invoices is the job nobody wants and everybody forgets. This scheduled function asks Zoho Books each morning which invoices are overdue and sends a reminder — but only at 7, 14 and 30 days, so your customers get a nudge rather than a daily email.

Before you start

• A Zoho OAuth connection named zoho_books_connection with ZohoBooks.invoices.READ and ZohoBooks.invoices.CREATE.
• Your Books organization ID.
• A scheduled function in Creator set to run once daily. Do not attach this to a form workflow.

Deluge function

// ============================================
// SCHEDULED FUNCTION — runs daily
// Send Zoho Books payment reminders on overdue invoices
// ============================================

void Books.sendOverdueReminders()
{
    ORG_ID = "YOUR_BOOKS_ORG_ID";
    CONN   = "zoho_books_connection";

    // 1. Ask Books for overdue invoices only
    criteria = Map();
    criteria.put("status","overdue");

    resp = zoho.books.getRecords("invoices",ORG_ID,criteria,CONN);
    invoices = resp.get("invoices");

    if(invoices == null || invoices.size() == 0)
    {
        info "No overdue invoices today.";
        return;
    }

    sent = 0;
    for each inv in invoices
    {
        invoiceId  = inv.get("invoice_id");
        daysOver   = inv.get("days_overdue").toLong();

        // 2. Only nudge on a schedule: 7, 14 and 30 days
        if(daysOver == 7 || daysOver == 14 || daysOver == 30)
        {
            // No native task for reminders - call the endpoint directly
            reminderResp = invokeurl
            [
                url  : "https://www.zohoapis.com/books/v3/invoices/" + invoiceId
                       + "/paymentreminder?organization_id=" + ORG_ID
                type : POST
                connection : CONN
            ];

            info "Reminder for " + inv.get("invoice_number")
                 + " (" + daysOver + " days): " + reminderResp.toString();
            sent = sent + 1;
        }
    }
    info "Reminders sent: " + sent;
}

Notes

• Filter server-side. Passing status: overdue lets Books do the work. Fetching every invoice and filtering in Deluge burns API calls and will time out on a real ledger.
• The day gate is what makes this humane. Without the daysOver check, every overdue customer gets an email every single day until they pay. That is how a helpful automation becomes a complaint.
• Reminders have their own endpoints. POST .../paymentreminder sends one, GET the same path previews the email body, and there is a bulk variant at /invoices/paymentreminder. None have native Deluge tasks.
• Paginate on real data. Books pages its list endpoints. Past a couple of hundred overdue invoices you need to walk page until page_context.has_more_page is false.