Thank you for your question, and thanks for using JustAnswer.com.
Unfortunately, you're in a situation where the IRS is correct. You needed to have a letter dated by the due date of the return from the charity confirming any donation over $250. Since you did not have that letter, the IRS will disallow the deduction.
So, in my opinion, it's not worth asking the girl scouts for verification because they are not going to write you a letter and back date it nearly two years ago.
You can CLICK HERE
for IRS guidance on this topic.
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You should be aware that your question, as with most tax questions, can never really be answered completely...addressing all the permutations; that is because of the many assumptions that have to be made I have done my best to determine what I think you are asking and answering it in the most direct and understandable manner possible. If, however, after reviewing the questions, you have any uncertainties or further questions, please do not hesitate to ask.