As others have said, you have to cross reference different tracking platforms: Shopify, Meta ads manager, GA4.
They will never match perfectly. Even big brands selling hundreds of orders accept this truth.
You just make projections, averages, and use heuristics to determine a trend, to determine if an ad is working.
Now, the easiest way to cross reference things is to do it with AI.
I use Claude Code, and I connect Shopify, Meta ads manager, Omnisend (the email marketing platform), GA4, Google Ads, etc.
Claude can read into all of them, precisely, at the same time, and analyse the data and especially the trends - which is, often, what we humans miss - we can’t extrapolate all the numbers in our own head/memory, but Claude can - and it will interpret everything for you as well, and can spot trends that we wouldn’t even notice.
You will certain numbers coming from / reported by / attributed to each platform. But some of the orders will never get attributed to any platform: either because they can’t be tracked (due to ad blockers, cookies, etc.).
But then you also have your total marketing spend across all platforms, which you know what it is, and your total revenue. Some of that spend will not get attribjted to any orders.
That’s where you enploy MER (total Marketing Efficiency Ratio) in the form of total ROAS, besides the ROAS of each ad platform. That should also guide you overally if your total comlany spend is efficient overall.