printing invoices in ASP.net web application without printer selection dialogue

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C#

ASP.NET

printing

I am working on a asp.net web application for INVENTORY MANAGEMENT SYSTEM.
I need to print a invoice/bill on every transactions. i have created code for preparing the bill in a gridview. What i want now is to print the content of that gridview(bill) on button click.
I have tried using the javascript function

function printPartOfPage(elementId) {
        var printContent = document.getElementById(elementId);
        var windowUrl = 'about:blank';
        var uniqueName = new Date();
        var windowName = 'Print' + uniqueName.getTime();
        var printWindow = window.open(windowUrl, windowName, 'left=50000,top=50000,width=0,height=0');

        printWindow.document.write(printContent.innerHTML);
        printWindow.document.close();
        printWindow.focus();
        printWindow.print();
        printWindow.close();
    }

which does print the content of the GRID but it prompts user to select the printer.
I want the bill to be printed immediately on button click through the default printer.