Greeting cards aren't the only convenience items grocery stores mark up. Many nonfood products like office supplies and light bulbs cost more at the supermarket than they would elsewhere. That's because supermarkets know that you'd rather make all your purchases in one place than run around town to get everything you need.
Sometimes it's actually not worth making the extra trip to another store to pick up compact fluorescents or motor oil, if gas prices are high. The bottom line is that you have to decide what's worth more to you -- convenience or saving money. Or, get out your scissors and look for coupons for convenience items at the grocery.