JCOM 1GB In My Room Internet, is it a fixed monthly fee or per usage?

Hello guys, I hope this question isn’t repeated, I searched but didn’t find an answer here.

I am new here in Japan, so I am wondering whether the monthly fee of J:COM In My Room 1GB package is fixed or it will change depending on the usage, they contacted me today about it and I decided to try it, but I am worried there are hidden fees or something.

Thanks in advance!

Edit: I see people downvoting already, I am sorry if this is a dumb question but I did search on the website and I found nothing, I wanna ask about the experience of people rather than ask the company itself.

by ThePirateKiing