Enterprise Rent-a-Car, responding to concerns by Engadget, made the following statement:
"[Enterprise] does not plan to charge customers for bringing back EVs without a full charge. Enterprise is installing charging stations at locations that will offer EVs, and plans to charge the vehicles once they're returned."This would really be more of an issue with a full EV, like the Nissan Leaf. Realistically, the Volt is a plug-in hybrid, since it has a gasoline tank as well as a battery, that's where any concerns over extra charges should come in.
On the other hand, this statement is pretty much a blanket statement, and should apply to full EVs as well, so good for Enterprise.