An even less intrusive solution are magnets which open a whole other world of possibilities as they are easily removed. If you have employees, agents, or brokers consider offering incentives for promoting with vehicle magnets possibly for a week or so at a time.
You’ll find many cheap solutions available but if you want them to last for any substantial amount of time you might want to go with higher quality. You can usually get square or circle magnets for a set price. Die-cut magnets, which are simply custom shapes will run you more. For a good car door size I’d expect to pay up to $100/each.