Shopify makes it very easy to install three things that all report the same purchase. Nobody notices, because the number goes up and up is good.
The ten-minute check
Pick last month. Open Google Ads and note the conversion count for purchases. Open your Shopify admin and count orders attributable to Google for the same window. If Google's number is meaningfully higher, you have a counting problem, not a performance one.
The four causes, in order of frequency
1. Multiple tags firing
The Google & YouTube channel app installs a conversion. Someone also pasted a gtag into theme.liquid. GA4 is imported as a third. One purchase, three conversions, 3x phantom ROAS.
2. Add-to-cart counted as a primary conversion
The account then optimises for people putting things in carts. That is not a business. Add-to-cart belongs as a secondary metric, never as the thing you bid toward.
3. Conversion value missing or wrong
Purchases counted, revenue not passed — so target ROAS has nothing to work with. Or the value excludes shipping and discounts, quietly understating your best products.
4. Post-purchase upsells not counted
Your highest-value orders look like your average ones, so the bidding never learns which products deserve more.
Why this is worse than a reporting error
Smart Bidding optimises toward whatever you tell it is a conversion. Feed it triple-counted purchases and it will confidently buy more of the traffic that produced them, at bids justified by revenue that does not exist. You do not just misread the account — you actively train it to lose money.
You cannot scale what you cannot measure, and you certainly cannot scale what you are measuring twice.
Fixing it properly
- Consolidate to one source of truth for purchases. Delete the rest, do not just mark them secondary.
- Make purchase the only primary conversion, with revenue attached.
- Check the value matches the order total you actually bank, after discounts.
- Account for consent banners — if a third of conversions are blocked, the bidding reacts to that as if demand fell.
- Re-verify monthly. Themes and apps break tags without telling anyone.
More on this in Google Ads for Shopify, or ask us to check yours — it is the first thing in the teardown and it is free.