Make API call to 3rd party app upon successful transaction payment

See NewBook PMS topic, would have everything you need on there to do yourself.

Personally i would consider that you post to api and Mark paid on success responce rather than blindly post. Or post days sales at end of day.
My topic covers both as cash sales are synced behind the scenes and room posts are posted there and then.

This topic is from scratch me learning scripts etc so should cover basics through to completion.