You should pay it online if you can so you are sure it is paid by 05/17. In fact, if you make a payment online, the extension is automatically filed. See IRS note below.
'''You can also get an extension by paying all or part of your estimated income tax due and indicate that the payment is for an extension using Direct Pay, the Electronic Federal Tax Payment System (EFTPS), or a credit or debit card. This way you won’t have to file a separate extension form and you will receive a confirmation number for your records.''''
https://www.irs.gov/forms-pubs/extension-of-time-to-file-your-tax-return
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