Dear …

I am writing to you today to thank you for looking after __(task you asked of them)__. When I called to ask this favor of you, I never expected that you would take care of it personally, and so quickly for that matter. You really came through for me and I am grateful.

It’s not often that you see something like this happen. I certainly value the business relationship we’ve built together, __(Name)__, now more than ever. Please feel free to call on me anytime if, in turn, you ever need a favor. I would be more than happy to oblige.

Sincerely,