Shopify is sending all personal data to CloudFlare, CloudFront (Amazon) and Fastly, so 3 US companies. They could sign so-called "data processing agreements" where they promise to safeguard personal data. But the Shopify FAQ explicitly states that they are unwilling to do so.
As the result, Shopify is legally considered to not be processing data under the instructions of the shop owner, because they didn't sign the processing agreement. Instead, Shopify is legally considered to be the owner of the user data. And that's a problem with EU clients because they are a US company working with US CDNs.
Still, all of this could easily be rectified by Shopify if they would just care enough to sign the correct paperwork.