(incorrect: ) No special vAT declaration for financial transaction costs

Hello,

That's correct!

If Shopify were a financial institution (which it is not), then it would fall under non-VAT-liable services. In that case, they would not pass on the VAT to you because no VAT would be charged.
If they do not charge vAT because it is not subject to VAT, then you should not have had to submit a special vAT declaration, because there was simply no vAT.

However, Shopify is indeed a regular company and they do apply reverse charge vAT. They should actually charge vAT, but in the international context they simply transfer all vAT formalities to you. So you do indeed have to submit a special vAT declaration.