Billing and db usage billing issues (Hobby Plan)

Hello I am new to fly and bad English,sorry.

My billing have a big price item: VM: Additional RAM, I don’t know when it was added my project.
I wonder why my billing is so much.

I found and read this article:

The solved says because db 1GB is not for free tier,my db created tier was:
Development - Single node, 1x shared CPU, 256MB RAM, 1GB disk

But isn’t forever free 3GB volumes ? why this tier is not free?
If i misunderstood,please tell me and please how to reduce the db cost to free.

Thanks alot!