Comment by fuzzy2

That would be illegal. I doubt Shopify are to blame here, it's more likely one of the gazillion plugins that every shop uses was the vector. Either way, it's highly likely the shop owner is the data controller, from a legal perspective.

(Scenario: E-Mail address A with shop A, address B with shop B, then received a newsletter I did not subscribe to [already illegal] from shop B to address A. Only common data point: PayPal account.)