[JennysCool]

Unfortunately, such a sticker makes your car a sitting duck for keying and other nefarious vandalism.

Get a magnetic "sticker" for your bumper which gives your message to other motorists, but you can easily remove while car is parked.

[mc5cents]

Get a magnetic "sticker" for your bumper

Good idea except for the fact that car bumpers are made out of fiberglass or plastic these days.

On the other hand, if you own one of these it should work fine.