If I'm pre-paying a service each month and I cancel my card, getting threats about collections is the exact opposite of what I expect. What I usually expect is for the service provider to either cancel my service or put it in some kind of suspended mode and then follow up with me to see if I really meant to cancel. Good billing providers would actually do this automatically: http://chargify.com/features/dunning-management/
As an aside, half the purpose of Zipcar is that it isn't a critical account. It's an easy-to-use convenience that saves you from the complexities of owning a car or making arrangements with a traditional rental service.