The idea of a computer script looking through your email is, indeed, creepy. What’s even creepier, though, is the idea of inadvertently leaving a package in your apartment lobby, so everyone can see that you’re ordering new sheets. Or letting an opportunistic jerk snatch the external hard drive you ordered, because the FedEx delivery person likes to knock with the might of an infant mouse.
To my mind, Slice is a prime example of an up-front trade-off. You’re letting a computer see what you buy, but, then again, Gmail and Yahoo already see that and more as they customize webmail ads for you. In return, you’re getting push notifications on your phone when your package arrived in your town’s shipping center, so you know for certain whether to listen for the rumble of the package truck. Everything is up-front, understandable by humans, and easy to break off, if you decide you want Slice out of your inbox. Simple, single-purpose, and slick.