For some reason, this didn't make sense to me....
A Zipcar is cheaper during the week, and more expensive on the weekend.
A rental car is more expensive during the week, and less expensive on the weekend.
We need a car to get a boatload of groceries and basement improvement supplies.
It's curious that the better price for a Zipcar is during the week and the better price for a rental is on the weekend. Who knew this?