The general rule personal jurisdiction is that in order to sue a person who does not reside in a particular state, the defendant must have purposefully availed him/herself of the benefits of the state jurisdiction, such that it would be fair to force him/her to appear and defend in that state's courts. So:
1. If the contract was performed in California;
2. The contract was accepted in California (last person to sign, or orally agree);
3. You did not initiate contact and solicit the business with the customer while he/she was located in Washington; then
The plaintiff must sue in California. However, you can waive your right to challenge jurisdiction by responding to the lawsuit in any manner other than to challenge jurisdiction. If you raise any other issue, no matter how minimal, the Washington court will determine that you are asking the court to determine the matter, and you will be stuck in Washington court.
Your choice is to either ignore the summons and complaint entirely, and hope that you can later prove that you were not subject to Washington jurisdiction, if the customer tries to collect against you in California; or you can send a letter to the court stating that you are writing for the sole purpose of challenging Washington jurisdiction, and then explain that none of the factors described above demonstrate that you have purposefully availed yourself of the benefits of Washington jurisdiction, and so the court should quash the summons and dismiss the action against you.
I hope I've answered your question. Please let me know if you require further clarification. And, please provide a positive feedback rating for my answer -- otherwise, I receive nothing for my efforts in your behalf.
Thanks again for using Justanswer!