Although it is absolutely fine to keep few things, for example, ash tray, maps, cigarettes lighters, etc. in your car when you are handing it over to the auto haulers for shipping, there are few reasons due to which you should resist to include any extra things (like, expensive and heavy items) in your car before the international car shipping company ships it.
First of all, there is always a change for your belongings to get stolen from the car during the time when the car carrier is stationed in-between the way. On the top of that, majority of car shipping companies do not have the license or any insurance policy that could protect you regarding your personal stuffs kept in your car. Due to this reason, the RV shipping company may not agree to ship your car in the first place till the time you take out all your personal things.
Moreover, there is a limit to the maximum amount of weight allowed, which should not exceed when shipping a vehicle. It is obvious that the extra items in the car will add to the weight and exceed that weight limit, causing you penalty or any undue deferment. Therefore, if you really want to ship any added stuff in your car, it is recommended that you should declare it to the international car hauler so that they know it before the transportation.