Replace the shipping fees on a Classic storefront with a flat-rate shipping fee.

flat-rate-shipping.html

<!-- BEGIN Use flat-rate shipping. See https://help.salsalabs.com/hc/en-us/articles/360001494394 -->
<script type="text/javascript">

// You configure this.  Change 10.50 to your shipping rate.  Please
// do not disturb the punctuation.

var flatRateShippingFee = 10.50;

document.addEventListener("DOMContentLoaded", function() {
if (RegExp('/shop/checkOut.jsp\\?storefront_KEY=').test(window.location.href)) {
	var e = document.querySelector("#shipping");
	if (e != null) {
		e.value = flatRateShippingFee.toFixed(2);
		e = document.querySelector("#sub");
		var sub = parseFloat(e.value);
		var total = (sub + flatRateShippingFee).toFixed(2);
		document.querySelector("#amount").value = total;
		document.querySelector("#amountDisplay").value = total;
	}
}
});
</script>
<!-- END Use flat-rate shipping. -->