I’ve got trial for mathematica online and all I can do is making windows with text “To access this feature, subscribe to a plan.”. I can’t make any document. Is that all what could make me pay $300?
What URL are you going to? I’m not familiar with Mathematica Online trials myself, but how did they give you the trial? Did they give you a key of some kind?
I would recommend contacting Wolfram Customer Service or Technical Support:
Please let them know what information you have and how you are trying to get to mathematica online.