TurboTax cannot mail or email your tax returns. You can download the PDF of your tax returns yourself.
Assuming the you have either paid the TurboTax online account fees with a credit card or have filed the 2018 federal tax return and it has been accepted by the IRS -
To access your current or prior year online tax returns sign onto the TurboTax website with the userID you used to create the account - https://myturbotax.intuit.com/
Scroll down to the bottom of the screen and on the section Your tax returns & documents click on Show. Click on the Year and Click on Download/print return (PDF)