Thank you -- do not send an amendment as that will just further confuse the issue.
What you need to do is to show them records - Make a spreadsheet showing the full income, and then show what was deposited into your account if it was just the net amount, or show where you paid groupon.
In the future, to avoid this -report the full amount, and then have an expense to show what you paid to groupon. The issue is simply a computer matching issue, and once you get it situated, you won't owe anything unless you did not previously pay self employment taxes.
There is a 1-800 number on your notice; call that and let them know that you will be sending this back. Make sure everything is as organized as possible.
If they continue to give you headaches, contact your local Taxpayer Advocate. They are the voice of the IRS and are independent of the IRS and are there to help individuals deal with the IRS when the IRS is being too bureaucratic.
Please let me know if you need anything additional. You are right, though - you will be able to deduct the cost of the groupon discount.
Thanks so much, and please rate so that I may receive credit for assisting you today.