Each email has a subject, a preheader and a message. Here is what to do with them.
Subject
Short and specific.
“Your payment has been received” is fine. “Payment confirmation for your Professional package” is better, because it tells them which of the things they bought this is about.
Avoid anything that sounds like marketing. This is a receipt, and it should look like one.
Preheader
The preview line that appears next to the subject in an inbox.
Almost everybody leaves this empty, and their email then shows a fragment of whatever the first line happens to be, which usually reads as nonsense.
Fill it in. It takes ten seconds and it is the second thing your customer reads.
Message
You can use bold text and formatting, as the plugin says at the top of the page.
Keep it human. Someone just gave you money, which is a small act of trust, and the email is your first chance to confirm they were right to.
Tell them what they bought, what happens next, and how to reach you if something is wrong. That is the whole job.
The failed payment email
This one is worth writing carefully, because it goes to someone who tried to pay you and could not.
Do not blame them. Most failed payments are a bank being cautious, not a customer being difficult.
Tell them what to do. A link back to try again, and an offer to help if it fails a second time.
Keep it short. They are already slightly annoyed.
A good failed payment email recovers sales. A bad one loses customers who were ready to buy.
Preview before you save
At the bottom of the page there is an Email preview, showing the subject and how it looks in an inbox.
The plugin is straight with you about it: the preview uses dummy data, an example customer, an example amount, and swaps in the real values when it actually sends.
Look at it. Then send yourself a test.