If you have letters or bills to mail, you might want to rush. Saturday is the last day when it will cost you 46 cents. Starting Sunday, you'll pay more -- 49 cents. On Sunday, the cost of mailing a post card will also go up to 34 cents, a 1-cent increase. The current rate hike brings a much-needed cash influx to the financially struggling postal service. Much of its cash problems stem from a congressional mandate to make annual $5 billion payments for future retiree health care benefits.