Thanks for replying 

I see 'All buyers' under recipients, do i just put that one username in there?

i just want to contact that one person, not everyone :)

If you look in your payments tab, you should be able to find the buyer's email address in there (clicking a result shows the associated Itch username). Keep in mind that you're not supposed to use this information for marketing or anything, however it does say that

The email address that itch.io provides, along with the email tool, are made available if you need to contact buyers with news related specifically to their transaction.

so I think in this case it would be ok? Someone from Itch would need to chime in on that but from an ethical standpoint it seems ok to me.