Police say the victim refused medical attention for the wound. An officer saw the injury, but a police report does not describe its size or severity.
Authorities say the woman is from Texas, but aren't identifying her.
Pittsburgh police spokeswoman Diane Richards says the woman was withdrawing money at 9 p.m. Wednesday when a man approached her from behind, put a knife to her neck and demanded money. She says she gave him $60.
The woman told police the robber then noticed the bumper sticker, punched her in the back of the head, knocked her down and carved a "B" on her face.