I typically tell clients to ignore civil demand letters. They are not actually due unless they get a civil judgment against you. The stores typically do not go that far. Further, it has no bearing on the criminal charges.
If this is your first offense, the District Attorney may send you a letter offering diversion which can resolve the matter without a conviction. You will have to make restitution which the $300 may be.
This response does not creat an attorney client relationship. In all cases, I recommend you seek a paid consultation with an attorney with expertise in this area.