Ask HN: I just got a paying customer. What to include in my welcome email?

I'm a solo founder and the only employee. What do I write in the email to make them feel good about the app and comfortable to reach out to me in case they have problems with the app?

I want to reduce the chance of them getting frustrated with the app and silently canceling the subscription. I would rather they reach out to me, so I can fix the problem or fill the gap they're facing with the app.

Wondering if you have suggestions for the email structure or copy, that you would appreciate as a customer?

I have sent such emails but limited success thus far.