ROAS Discrepancy on Shopify with GTM Server-Side and AfterSell Upsells

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Hi everyone,

I need expert advice on a tracking issue that started immediately after I installed the AfterSell app on my Shopify store.

Since adding the app, my reported ROAS in Meta has dropped significantly (showing 1.6 while my actual is 2.0). It seems the post-purchase upsell funnel is disrupting the standard tracking flow.

The Setup:

  • Store: Shopify

  • Stack: GTM Web (GTM-PR8KFDWN) + GTM Server (GTM-KCB26RQD) via Stape.

  • The Problem App: AfterSell (Post-purchase upsells).

What’s happening:

  1. Funnel Break: Before AfterSell, the customer went straight to the “Thank You” page. Now, they hit the upsell offer, and it seems the GTM Web Container either loses the session or fails to fire the final purchase event with the correct aggregated value.

  2. Data Leak: My Shopify Customer Events (Pixel) shows a “not subscribed to any events” warning, even though the Stape CDN script is present.

  3. Tag Placement: I’m also getting a “Tag not placed correctly / too low in the page” error in GTM, which I suspect is causing us to miss users who exit quickly after the initial purchase but before the upsell is processed.

Questions for the experts:

  1. How do I properly “subscribe” to AfterSell events within the Shopify Pixels (Customer Events) sandbox so they are sent to my Stape Server?

  2. Is there a recommended way to ensure the initial purchase and the post-purchase upsell are aggregated into a single purchase event before being sent to Meta CAPI?

  3. Should I move my entire GTM implementation into a Custom Pixel to fix the placement issue and ensure it works during the AfterSell redirect?

I’m looking for a “bulletproof” way to make sure that no matter what apps I add to the checkout, the Server-side tracking captures the final transaction value from the server, not just the browser.

Thanks in advance!