Forfeiting bail on a traffic matter is more commonly known as "paying the ticket." The forfeiture amount on your notice from the court is usually the standard fine the judge would charge if you appeared in court and entered a guilty plea or were convicted after a court trial.
How does a $20 fine turn into $151? With all of the additional fees the courts tack on, such as courthouse security fees and building funds. You can contact the clerk of the court to find out exactly what they add to get to that figure.