Is this homework? If it's not in the agreement (e.g. menu) prices, which you accept by ordering) - no, it cannot be added later without consent.
If it's homework, Avvo is not for homework.
The above is general legal and business analysis. It is not "legal advice" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.
You accept the gratuity term by placing an order, and you become contractually liable for that gratuity amount.
The information presented here is general in nature and is not intended, nor should be construed, as legal advice. This posting does not create any attorney-client relationship with the author (who is only admitted to practice law in the State of California). For specific advice about your particular situation, consult your own attorney. Sarkis Sirmabekian 818-473-5003.
I agree with Mr. Doland and wish to add a bit of extra information. There does not appear to be a clear California case on point. (If any other attorney knows of one, please post it). Amazingly, in New York, one court ruled that a mandatory "TIP" was not mandatory. However, if you agree, or if it says on the menu or it is announced before you order that a "Seating Fee" or "Service Charge" of a certain percentage will be added to the total of the bill, then the extra fee is probably owed. There probably is not a case on point in California because the amount is probably not worth fighting over. Also, keep in mind that in the New York case, the restaurant called the police and the patron was arrested for' theft of services'. [Ouch!] Even though the customer won, it probably cost many times the restaurant bill to win in court. All the best.
*Scott G. Nathan has been licensed to practice law in California since 1983. The information presented here is general in nature and is not intended, nor should it be construed as legal advice for any particular case or matter. This posting does not create any attorney-client relationship with Scott G. Nathan or my law firm. For specific advice about your particular situation, you should consult with an attorney immediately.