Simply click 'Settings' in the sidebar menu.

Once you're in the Settings section, choose the brand you wish to edit under the ‘Branding’ category of settings and go to ‘Payments’ tab.

You have 3 'thank you' messages which you can customise in Better Proposals. The first is the message when someone's signed a document. The other two are what we're covering here which is:

Your message will be displayed on the left side here:

It's always good to write something here explaining the next step. Try to keep it simple, you want them to pay, not bombard them with information. If you'll arrange a meeting after they've paid, tell them that. If you'll invoice them, tell them that. Just keep it chill.

Reassure them here that what they've done is a smart move. Words of encouragement here are good. "Congratulations", "Welcome aboard". 

You can do some clever things here like link them to Calendly or BookLikeABoss to book a call if that's the next step. Perhaps you want to collect some information from them, for that you could link them here to Typeform or something similar.

The most important thing is to be clear.