The real problem is Shopify is not the one who should receive this notice. They should act like a post office and pass it onto their users. Shopify should only do more if the poster does not respond in a reasonable amount of time, or if a court order is obtained. (Or of course the user takes things down). Of course a user responding to leave content up needs to provide their legal contact information so this can go to court.

But if they don't take it down as soon as they are "notified" then they lose the safe harbor protections. I find the DMCA enraging but the law is pretty clear. You incur some liability by not taking down immediately, and no lawyer or business is going to incur liability when they don't have to.

What I wrote is how the law should have been written (IMHO), not how it is. You are correct the law forces them to act as they do.