After you file

As long as the amended return is successfully e-filed and accepted, you would not need to mail the amended return in.  If you try to e-file and your amended return does not get accepted, then yes, you can mail the return as the IRS would have not received the amended return.  

As far as attachments, include any supporting statements that reinforce the numbers that changed from your original tax return.  In this situation, if the cost basis was originally reported incorrectly, you can include the statements showing the correct cost basis.  Also, make sure to list in the description the reason for the amended return.

Please see Instructions for Form 1040-X if you need addresses for where to mail your amended return.  You will find the addresses on page 11.  Please see the excerpt below for guidance on how to assemble your amended return if you end up mailing it in.