Filers who wind up with a tax bill instead of a return have the option of using plastic to settle up with the IRS. Although doing so is perfectly safe, it is not free. Even if the total credit balance is paid off before the next billing cycle, certain charges still will apply. They include third-party processing fees, which can tack on an additional 2.99 percent to 3.95 percent for using a debit card or 1.88 percent to 3.93 percent of the tax bill for using a credit card. That means coughing up anywhere from $18 to upwards of $40 more on a tax bill of $1,000. Taxpayers who use tax software to file electronically, meanwhile, should expect to pay 2.49 percent to 3.93 percent extra, not including a convenience fee of at least $3.95. Consumers can avoid the processing fees simply by paying in person or by mail.