I was deferred early action and was reading about letters of interest… would it be too late to send one and have it still be affective? Or would it seem like an afterthought? UChicago is my top choice, and I wanted to apply early decision but couldn’t because of the financial commitment. Thanks in advance for advice 
It won’t hurt if you send one right now, even if you are late.
Also explain why you were not ED.
is it better to send a paper letter or do it electronically? Do you email it to the regional admissions director if you do it electronically?
Either but electronically will be faster.