Your waiting time penalty claim would accrue starting from 72 hours after your final work day. You could get a penalty of one day of pay up to 30 days for every day they fail to pay your final wages.
Your employer should not have withheld your final pay check because of the key issue - they were obligated to pay your final wages regardless of any issue related to the key. Also, under CA law they cannot charge you for business related losses so you never should have paid the $50.
You would just need to submit your final pay stub. It sounds like the dates are not in dispute so your employer has liability. Employers who violate a basic wage law like this often violate other wage laws so you should consult an employment lawyer to see if you have any other claims.
Mr. Ferndandes is correct. Since you did not give at least 72 hour notice of quitting, your former employer was legally required to have your check ready for you to pick up at the usual place of payment no later than 72 hours after you quit, including all wages owed and any earned and unused vacation or PTO. They cannot hold your check because of ANY issue, including their allegation that you did not return a key. When did you advise them to mail your check and did they agree? An employment law attorney should be able to send your former employer a demand letter to get the waiting time penalties owed to you PLUS your attorneys' fees. If you have the envelope that the check was mailed in, that will help, along with a copy of the check, emails regarding the issue. 949-481-6909.
Please see the responses you received to your very similar question.
twitter.com/MikaSpencer *** All legal actions have time limits, called statutes of limitation. If you miss the deadline for filing your claim, you will lose the opportunity to pursue your case. Please consult with an experienced employment attorney as soon as possible to better preserve your rights. *** Marilynn Mika Spencer provides information on Avvo as a service to the public, primarily when general information may be of assistance. Avvo is not an appropriate forum for an in-depth response or a detailed analysis. These comments are for information only and should not be considered legal advice. Legal advice must pertain to specific, detailed facts. No attorney-client relationship is created based on this information exchange. *** Marilynn Mika Spencer is licensed to practice law before all state and federal courts in California, and can appear before administrative agencies throughout the country. She is eligible to represent clients in other states on a pro hac vice basis. ***