My state tax for 2013 turned out to be larger then was withheld. As a result, the extra state tax I paid was included (by Glacier software) into the "Itemized deduction" line of my 2014 federal return. Apparently, the IRS did not believe that I paid this extra state tax, and that I am qualified for that extra deduction: they sent me a notice that they want me to prove it. The same situation is described in this question.
How do I prove I paid that extra tax? Should I request some form from the state revenue service? Or should I just send to the IRS all 10 pages of my 2013 state tax return?