There are two problems: your failure to read, and their apparent attempt to send faxes that you did not receive.
You are not going to jail, but this could impact your credit.
I would report this to their better business bureau and to the attorney general of the local state,
I suggest that you contact the Better Business Burea and the Attorney General's Office, Consumer Affairs Division and explain your situation to them. The company may have disclosed their practices, but there are guidelines that must be met. Disclosure, to be legally adequate, cannot be obscure and difficult to locate. An attorney would need specific detailed information that would then be tested against required standards. In other words, you would have to hire an attorney to specificly challenge their practices. The company may rely upon heavy handed threats to dissuade people from chalenging their practices. I would strongly urge you to consult with the two agencies mentioned above before I paid them a penny. I would allow the credit card company to investigate and go through their evaluation process as well. If they wrongfully harm your credit, you can sue them for doing so.
I just went to their website and I can't believe how outrageous their terms are. They can say anything they want, but you should have nothing to worry about. Just the fact that their name is "one time fax" and they charge $27 to cancel an account is crazy and probably illegal. You should contact the Federal Trade Commission and the Massachusetts Attorney General's Consumer division. Do not pay the fees. I would sue them under chapter 93A, the Mass. Consumer Protection Act.
I use Efax in my law practice and I find them very dependable and easy to deal with.