I checked box c (should be box a) wrongly on form 8949 and e-filed. What's the best way to fix this problem?

You don't have to fix your tax return. One of the requirements to amend your tax return is that you made an error that affects your bottom line. You don't need to file an amended return if you correctly reported your cost basis and proceeds for your short-term transactions. Checking either box A or box C doesn't affect your refund (or amount due). 

• Form 8949 Box c is Short-term transactions not reported to you on Form 1099-B. Box A is Short-term transactions reported on Form(s) 1099-B showing basis was reported to the IRS. By sending in your Form 8949 and entering your 1099-B forms, you've informed send in your cost basis.

 

Thanks a lot. After e-filed, turbotax said it is not completed until I mail the document to IRS, and attached form 8453(which check box Form 8949). Do I need to mail form 8453 and supporting documents to IRS in this case? If yes, what are the supporting documents should I include in the mail, 1099-B forms?

Yes, you are required to send in additional information, if you did a summary in lieu of individual transactions. This means you can E-file but you have three days, from the date the IRS accepted your return, to yes, mail in the your 1099-B forms that you received from your financial institution.

 

There's no cover letter. Just print and mail Form 8453. Mail Form 8453, with your attachments to: 

 

Internal Revenue Service

Attn: Shipping and Receiving, 0254

Receipt and Control Branch

Austin, TX 73344-0254