Thank you so much for your letter of reference for the student who applied to our NSF REU program. It was so very informative. Knowing that the student's favorite activity is reading will truly aid our decision process. So much more than the information we requested, such as their research potential, ability to work with others, or even classroom performance. I can fully understand why you have given them the "strongest possible recommendation."
And of course you clearly understood that no apologies were necessary for sending in your reference letter two months past the deadline.