Is this how the “Free” Google API works

I have a Google Maps question. I needed to get a “working” API for this plugin, so I had to give Google a credit card for my account. From what I can tell, you get free use of the API as long as it isn’t a high-traffic site. They also give you a Free Trial credit of $300 which goes for any project there. As long as I don’t “upgrade” my account, the free trial credit will expire after 30 days, but I will still be able to use the Google Maps API as long as I don’t go over their quota? Is that right? If I do go over the quota, I will not be billed, but the functionality of the map will go down. I’m trying to make sure that I don’t get a weird bill out of the blue.

Thanks
Bryan