Earlier quoted context omitted.
StackOverflow and all the StackExchange family of sites are down. I suspect the lost productivity from that will be more costly over the whole economy than potential lost sales via Shopify. People can go back to shopify so those transactions not definitely blocked for ever, any time "lost" due to reference resources being unavailable can't so easily be claimed back.
I don't think you understand how ecommerce works. A very significant amount of people won't go back. It's why the most effective marketing campaign by far is retargeting those people to convince them to come back. Unfortunately that's not possible in this case since you can't track the users as the site is unusable.
So they didn't need what they were about to purchase and saved their money. Doesn't sound like a net loss to me.