Certified. In a few days, I look up the tracking code and print out a page that shows when the package was received by the IRS. I file a 1040NR and don't know how to file it electronically: TurboTax doesn't support 1040NR.

Turbotax had me snail mail federal and both state returns because we moved mid-year and my spouse had two W2s from the same employer.

First class.