It's the 1st link, not the 2nd, which only covers adverts. It looks like you are claiming:
They failed to provide a written estimate;
After tear down, they failed to update the estimate in writing;
At any time, they had no written or verbal authorisation to any cost beyond the price agreed ($2,500).
Whatever you do next, take witnesses, someone who is mechanically competent to assess what has been done and keep records of the conversation, ask to record it on your phone. Rehearse how you will make your case to them, anticipate their arguments and your responses. At some point make it clear what the contract was for and how much and that you will honour the contract for that work at that agreed price, once you are happy, on inspection, that the work has been competently carried out. Make sure you and they have a record of this that you can refer to later. Keep your cool under all circumstances.
If they are legit, you'll get somewhere. If they're crooks, you have to decide whether you spend more money to have your day in court; or walk away and write it off to experience. If they make you an offer, take time to think about it.
If you've behaved as you've told us, I'm very sorry this is your introduction to biking. It isn't normally like this!