Voluum and Shopify events tracking integration code. It's based on native Shopify Customer Events, and it really works without any issues.

shopify-customer-events-pixel.js

// https://help.shopify.com/en/manual/promoting-marketing/pixels/custom-pixels/code
// https://shopify.dev/docs/api/web-pixels-api/standard-events

function getPostbackURL() {
	try {
		const utmMedium = localStorage.getItem('utmMedium');
		const xoClickId = localStorage.getItem('xoClickId');
		if (utmMedium === 'xo' && xoClickId) {
			return 'https://your-voluum.domain/postback?cid='+xoClickId;
		}
	} catch(e) {
		console.log(e);
	}
}

analytics.subscribe('page_viewed', async () => {
	try {
		const url = new URL(window.document.location.href),
		utmMedium = url.searchParams.get('utm_medium'),
		xoClickId = url.searchParams.get('utm_term');
		if (utmMedium === 'xo' && xoClickId) {
			localStorage.setItem('utmMedium', utmMedium);
			localStorage.setItem('xoClickId', xoClickId);
		}
	} catch(e) {
		console.log(e);
	}
});

analytics.subscribe("product_added_to_cart", async () => {
	if (getPostbackURL()) {
		const timeStamp = Math.round(Date.now() / 10000);
		navigator.sendBeacon(getPostbackURL()+'&et=add_to_cart&txid='+timeStamp);
	}
});

analytics.subscribe("checkout_started", async () => {
	if (getPostbackURL()) {
		navigator.sendBeacon(getPostbackURL()+'&et=begin_checkout');
	}
});

analytics.subscribe("checkout_completed", async (event) => {
	if (getPostbackURL()) {
		const checkout = event.data.checkout,
		totalPrice = checkout.totalPrice.amount,
		priceCurrency = checkout.totalPrice.currencyCode,
		orderId = checkout.order.id;
		navigator.sendBeacon(getPostbackURL()+'&et=purchase&payout='+totalPrice+'&currency='+priceCurrency+'&txid='+orderId);
	}
});

Read full article about Voluum and Shopify Events Tracking Integration before using this code.
Watch short video How to add tracking code to your store and setup "Customer events pixel" options.