I appreciate your answer, but again this is vague:
"mail in a check and pay when you can"
Is this full amount due on April 15 ("mail in a check") or is it due/can i get it due at a later date ("pay when you can")
If you pay after April 15 they will send you a bill for interest. You can request an installment plan. I don't know how that works. Maybe when you file at the end it will give you that option.
you can request an installment plan when you file, but it is not an extension of time to pay. you still pay any penalty and the interest charged to your balance until it is paid off.