I’ve added a tax total line on the cart.liquid page with:

<div class="total-label">GST:</div>
<div class="total-value-gst">{{ cart.total_price | times: 0.10 | money }}</div>

I’ve changed the class for the value to .total-value-gst to ensure that on update of quantities, the JS doesn’t override it to the total price. Inside of polaris.js I can see:

initCartTotalPriceUpdate: function (total_price) {
	var cart_form = jQuery('form.cart');
	var total_varies = cart_form.attr('data-total-varies');
	var new_price;
	if (total_varies == 'true') {
		var compare_price = 0;
		jQuery('.cart-items form').each(function () {
			compare_price += parseInt(jQuery(this).attr('data-line-price-compare'));
		});
		new_price = "<span>" + Shopify.formatMoney(total_price, window._money_format) + "<s>" + Shopify.formatMoney(compare_price, window._money_format) + "</s></span>";
	} else {
		new_price = Shopify.formatMoney(total_price, window._money_format);
	}
	cart_form.find('.total-value').html(new_price);
},

… which is the function I think that updates the price on qty update. How can I modify it to also update the GST/tax (it’s always 10% of the total price).

I realise this is out of scope of normal support, so any hints you had like last time would be much appreciated. Thank you!