How do I send thank you notes to my customers as soon as they pay for an invoice?

Manual Mode:

When you record payments for your invoices, you have an option called Send a “thank you” note for this payment below the notes section of the payments page.

By checking on this, you can send a thank you note on receiving a payment.

Automatic Mode:

You can also automate sending thank you notes to your customer. To do so:

Screen shot of Invoices in preferences