> Ultimately they sent me a bill for the cost of the rental minus the gas and threatened to take me to collections if I didn't pay it.
This is the always-necessary reminder that credit card chargebacks do not in fact relieve you of the duty to pay for services that you did receive. A chargeback may be necessary to get the merchant's attention, but you do need to be careful to actually pay them the correct amount in the end.
Maybe still a win in the end because the incorrect charge was removed, but I get really uncomfortable with how casually people recommend chargebacks on this forum. They're not intended to get you out of charges that the other party can prove you did agree to pay.