Hopefully, this issue will go away soon.

I'm uniquely positioned to talk about this as I provide a free Shopify Shipping App that I'd imagine would use the same APIs as the "third-party international shipping plugin" you talk about.

Shopify has identified 3rd party app server capacity as an issue with "flash sales", and this is why they've been throwing a bunch of dev time in getting web assembly to run out of the browser and on the server. Their aim is that they'll run the App's code on the app dev's behalf. Web assembly was chosen for sandboxing.

Here ends my knowledge!