I highly recommend the IRS website - direct pay.  using your bank account is free (you can use a debit or credit card for a fee) and you can even specify a future date for the withdrawal to be made. The IRS may actually withdraw the funds a couple of days later but you get credit for it on the date you specified as long as it is a banking day.  if you select a non banking day - holiday, saturday, sunday, the credit will be the next business day.    at the end the website will allow you to print a receipt.   the advantage is proof of payment and no chance of it getting lost in the mail.  

https://www.irs.gov/payments

the website offers other ways to pay, too

for direct pay 

you'll have to provide identity verification which is info from any return filed in the last six years. 

also make sure to enter correct routing and account number