Shopify's checkout runs on Shopify's domain and in a sandbox, which is where most ad tracking breaks. BrainyMetrics covers it from both sides. A theme snippet captures the cart token on your store, and a custom pixel covers Checkout Extensibility, so the order is tied to the browser that placed it.
Webhooks for order creation, payment, cancellation and refunds bring in the exact amounts, line items and customer details. Post-purchase survey answers saved as order note attributes (like "How did you hear about us?") are recorded as the buyer's self-reported source.