I may be in the minority here, but while I empathize with you over the indignity of it I don't think this would be a slam-dunk case for you unless their agent just doesn't bother to show.
You paid them to serve ads. They presumably served the ads as promised, and your store is currently benefiting from the ads. "But I didn't want them" doesn't get you far when you previously signed a recurrent-billing contract.
If this gets all the way to a judge, on paper, it'll look like you just failed to stop service. You'll argue it was impossible. They'll reply with "you could have gone to [some form on some obscure page]" to cancel. The icing on the cake will be "you would have known if you read the terms and conditions you signed."
Skip that humiliation and re-read the terms and conditions to see if there's some specific rain dance you have to do to end the contract.
If nobody here can work some magic on your behalf, I would suggest you cancel your credit card and cut your losses ($300 is less than a speeding ticket or runaway AWS bill). The time involved in filing and the mere cost of parking at the courthouse once or twice is going to claim a chunk of that anyway, if you win.
If you're really bitter, file chargebacks. You can always bypass blacklists by playing at synthetic identity fraud (start using androgynous nicknames for yourself, typoed versions of your last name and addresses of friends/family on all future applications for anything to fuzz the credit bureaus).
There's a reason everything is now subscription-based with an obfuscated cancellation process-- we've found a way to institutionalize negative option billing. Welcome to America.
ed: I started poking around at it for you just for fun. I lost faith in the legal system so long ago that I forgot all about the joys of arbitration! You definitely wouldn't win at kangaroo court.
https://www.facebook.com/legal/commercial_terms
> You agree to arbitrate Commercial Claims between you and Meta Platforms, Inc. This provision does not cover any commercial claims relating to violations of your or our intellectual property rights, including, but not limited to, copyright infringement, patent infringement, trademark infringement, violations of the Brand Usage Guidelines, violations of your or our confidential information or trade secrets, or efforts to interfere with our Products or engage with our Products in unauthorized ways (for example, automated ways). If a Commercial Claim between you and Meta Platforms, Inc. is not subject to arbitration, you agree that the claim must be resolved exclusively in the U.S. District Court for the Northern District of California or a state court located in San Mateo County, and that you submit to the personal jurisdiction of either of these courts for the purpose of litigating any such claim.