College Application Fee Waiver

I got a fee waiver in the mail from Northeastern University, and I submitted my Common Application Early Action saying that I have a fee waiver for them. The university is asking for official documentation that I have a fee waiver. I have the letter they sent me saying that I have a fee waiver, but what am I supposed to do now? Should I send this letter back to them, or is there a different way I am supposed to go about this situation?