If you make an estimated or extension payment greater than $20,000 or have taxes owed to California greater than $80,000, you are required to pay all California taxes (including estimated taxes) electronically or you'll be charged a 1% penalty. The first payment that would trigger the mandatory e-payment requirement doesn't have to be made electronically. This applies to all subsequent payments after either of these thresholds are met, regardless of tax type or taxable year.
For more information on this requirement or for how to make electronic payments, go to www.ftb.ca.gov . You can also call 1-800-852-5711 for any questions you have.