Add a thousands separator to a total with Javascript or jQuery?

This will add thousand separators while retaining the decimal part of a given number:

function format(n, sep, decimals) {
    sep = sep || "."; // Default to period as decimal separator
    decimals = decimals || 2; // Default to 2 decimals

    return n.toLocaleString().split(sep)[0]
        + sep
        + n.toFixed(decimals).split(sep)[1];
}

format(4567354.677623); // 4,567,354.68

You could also probe for the locale's decimal separator with:

var sep = (0).toFixed(1)[1];

The $(this).html().replace(',', '') shouldn't actually modify the page. Are you sure the commas are being removed in the page?

If it is, this addCommas function should do the trick.

function addCommas(nStr) {
    nStr += '';
    var x = nStr.split('.');
    var x1 = x[0];
    var x2 = x.length > 1 ? '.' + x[1] : '';
    var rgx = /(\d+)(\d{3})/;
    while (rgx.test(x1)) {
        x1 = x1.replace(rgx, '$1' + ',' + '$2');
    }
    return x1 + x2;
}

Use toLocaleString()
In your case do:

return "Total Pounds Entered : " + tot.toLocaleString(); 

toLocaleString() method's syntax looks like:

toLocaleString()
toLocaleString(locales)
toLocaleString(locales, options)

If your browser can't work with toLocaleString() you can try use locales argument, for example:

var number = 123456.789;

// German uses comma as decimal separator and period for thousands
console.log(number.toLocaleString('de-DE'));
// → 123.456,789

Full documentation available here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number/toLocaleString