Despite a protest from a coalition of consumer groups, America Online says it will stick with its plan to charge mass e-mailers a fee for guaranteed delivery of messages to AOL subscribers. AOL plans to offer the service within 30 days, AOL said. "Mark it on your calendars," AOL spokesman Nicholas Graham was quoted as saying in that would be a burden on non-profits and other groups that can't afford to pay the toll. The fee per message would likely be about one-quarter of a cent, according to Goodmail Systems, the company that offers the certified e-mail service. AOL says the plan will help reduce spam, which has become a universal plague to e-mail inboxes.