Charge for going over 50mb in free tier

I want to make sure, if I go over the free tier storage on for like 50mb (anything less than a GB)
1- Do I get charged for the whole GB or just for the 50mb that I went over?
2- If the storage that Im using changes (example, I had 10gb on Dec, but on Jan I removed 5gb) will I still be charged for the 10gb or only for the 5gb that are currently on the app?

I want to make sure I understand the pay-as-you-go because it will fit my app necessities, thanks team