You must still pay your taxes on time, regardless of when you get the paperwork turned in. If you do not pay your taxes by the deadline, you will be subject to various penalties depending on how much you have left to pay. For instance, if 10% of your taxes are remaining to be paid, you will have to pay a late payment penalty. The IRS can also charge you interest on the amount you have left to pay. And if you haven’t even filed your tax extension by the deadline, you will also be subject to a late filing penalty, which—you guessed it—will cost you more money.
Once again, we repeat: Pay your taxes on time. A tax extension gives you a grace period to file the paperwork, not to pay your taxes.