I'm sure she's capable of understanding that on her own. If you'd like to cut it, I recommend pulling some numbers:
- how much stuff you ordered (esp. how many were under the $35 or whatever order limit)
- how often you actually needed something faster than standard shipping (usually 3-5 days)
- how much you paid for the service
With that, you could make a case for dropping it. You could also agree that if there are problems, you can always sign up again. Then it's on you to anticipate things you'll need and order in advance. I am proactive about keeping stuff on hand, so it's not an issue for us. That's how I convinced my wife we don't need it, and we haven't had it for a couple years now
That said, get a free Amazon Prime trial every 6 months or so, so we often sign up around Christmas, then cancel.