There are few things more frustrating than getting a notification that a package couldn't be delivered. It requires far too much work to get it back on track. But when you have a package that needs to be delivered, you'll do what it takes. That's what this phishing scam aims to take advantage of.
In order to get the package delivered, you have to reschedule the delivery. As you can see, that URL does not lead to the UPS site, but a clear credential harvesting page.