California Business & Professions Code 9884.9 states the following: "The automotive repair dealer shall give to the customer a written estimated price for labor and parts necessary for a specific job. No work shall be done and no charges shall accrue before authorization to proceed is obtained from the customer. No charge shall be made for work done or parts supplied in excess of the estimated price without the oral or written consent of the customer
that shall be obtained at some time after it is determined that the estimated price is insufficient and before the work not estimated is done or the parts not estimated are supplied. . ."
A motorcycle is defined as a automobile for purposes of this section. Therefore, the repair facility is barred from collecting anything if there was no written estimate. If they gave you a written estimate for $300 they can charge more if they called you and you orally agreed. If they did that then they must make a notation on the written estimate or invoice of the date and time they placed the call. Assuming there was not a written estimate, they may try to recover the $300 under a quantum merit argument but I would argue that the statute is not forgiving and it is a consumer protection statute. And there is no way they can collect anything over and above $300. I hope you did not sign a blank estimate. You need to report them immediately to the Bureau of Automotive Repair (BAR).