Its best you get advice like this directly from an attorney. Most offer free initial consultations. You can find a good divorce lawyer in San Antonio at the link below. I also put a few links for California Lawyers, Legal Advice, and Legal Articles.
To sign a "blank" contract only to be filled out later is just plain stupid.
Why could you not just pay for 1 month vs signing a contract?
Are they deducting the fees from a credit card or your bank? If so, you need to dispute this in writing. But it sounds like this has been going on for several months, so could be hard pressed to state you did not want, but for 1 month only.
If they have taken only ONE extra payment, meaning 2 months, then dispute this and make sure that either your credit card or bank does not take # 3. But if 3 or more, even with the other details of expiration, zip code is meaningless.
You could seek the advice of a lawyer for your legal options. Many will give you 1 free consult, and if this contract is valid, then you will be on the hook for 2 years.
I did google to see if other complaints and found none on their contracts.