Simply follow the notice instructions carefully ... send them exactly what they are asking for so they can finish processing the return.
Why they want it now is a mystery but this seems to be a new trend for the IRS to ask for supporting documents prior to releasing the refund instead of auditing the return 2 years later ... must be part of the new system to keep from sending out incorrect refund amounts to start with instead of trying to get the money back later.
https://www.irs.gov/individuals/understanding-your-letter-0012c
