You could print a QR code on the ad rate card that leads to a floating BTC/USD price on your website in addition to printing the USD price on the card.

Most businesses that take bitcoin do so through an intermediary that handles the billing and USD to BTC conversion rate. Since most of these businesses are online only intermediaries usually provide a widget that shows prices in BTC using current market rates. Bitpay it a popular one but I'm not sure if they let you keep the BTC, I think they cash you out in USD daily.