Struggling with Pricing

How should I flat-rate vehicle wrap designs? I'm a full-time packaging designer (bags, boxes, etc.) and I've been taking on commercial vehicle wrap work on the side. I charge flat-rate because I hate tracking hours and like juggling multiple projects. Most jobs are "here's an AI mockup, make it print-ready," and the client usually supplies most/some of the assets. I'm not experienced enough with wraps to know what I should be charging for that. What would you charge for this kind of job? Should it vary based on how many assets are provided?