file by mail and pay electronically?

[arqchicago]

Hi, I am filing my federal tax return by mail due to my situation (NRA spouse). However, I would like to pay online using direct pay. If I pay online, what should I include for payment confirmation with my return that is mailed to IRS? 

[DoninGA]

You can but it is not required.  The payment you make on the IRS website will be in your IRS tax account - https://www.irs.gov/payments/your-online-account