i would pay using the online method if you have a bank account. you can use the IRS Direct Pay to make an ACH withdrawal from the account - fee free or you can use a debit or credit card - fees apply. you can specify a payment date that is up to 30 days in the future. you can request an email confirmation of what you did or print out a receipt.
the next payment is due 9/16/2019. so if you use the mail, it must be postmarked on or before that date. I would also highly recommend using a method whereby you can track the mailing. Sometimes thing get lost in the mail. if you use the online method , the actual withdrawal may take place up to 2 business days after the date you specified. However the iRS regards the payment made on the date specified assuming there's enough money in the account.
