I am an attorney licensed to practice law in California. Do not take this as creating an attorney-client relationship. Here is my opinion.
Although it is theoretically possible for Target to change its mind and criminally prosecute you, that is extremely unlikely. The chain has a very thorough loss prevention department. They don't tend to second guess decisions about prosecution. They don't generally lie to someone and say they are not going to prosecute and then file a criminal complaint. Havng said that, Target does prosecute often and you were lucky that the person who caught you let you off easy.
In California the civil demand letter can be between $50 and $500. Expect something on the high end. Although civil enforcement actions in California for failure to pay a civil demand are rare, the risk to you if you do not pay is that it can be increased by attorney fees. The bill could amount to several thousand if you do not pay it promptly. In your case, where your guilt is not an issue, you should probably pay it.
Your name may also have been entered into a retail theft database. If so, other potential retail employers can check on this, so don't expect to get a job in retail for at least 7 years.
You may also have been given a store ban, ordering you to stay away from Target for a year or more. If so, respect that. If you violate it you can be prosecuted for criminal trespass even if you are not doing anything else wrong.
And stop shoplifting. Although I have never handled a shoplifting case in the context of California's three strikes law, I understand that shoplifting can count as one of the strikes. You have been lucky so far. Don't expect that to continue. Keep doing this and you could wind up with a lengthy prison sentence.

