@Dimitris211 ,
Advertising might suggest that you are working for the car rental shop.
But if you recommend, that's another thing. As hosts, we are supposed to recommend businesses around our properties, like restaurants, shopping, attractions, bars, museums, hairdressers, etc.
Our guests appreciate our recommendations because it saves them time. However, we ensure that our recommendations are free of interest besides providing information to the guest.
So, to answer your question, if the car rental is a recommendation, go for it. I am sure your guests will appreciate it. But don't advertise and persuade your guests because if something goes wrong with the car rental, it will affect their reviews of your property.