function calculate();
{

        var mugPrice = 5;
        var keyringPrice = 2;
        var tshirtPrice = 15;

        var numMug = document.getElementById('numberOfMugs').value
        var numTshirt = document.getElementById('numberOfTshirts').value
        var numKeyring = document.getElementById('numberOfKeyrings').value

        var totalpay = (numMug * mugPrice) + (numTshirt * tshirtPrice) + (numKeyring * keyringPrice);

        document.getElementById('totalPrice').value= totalpay;
}

<button class="calcButton" onclick="calculate()">Calculate Order</button
<h3>Total price: <input type=text; id="totalPrice" disabled /> </h3>