I think it would really depend on the specific flyer builder website and if they have a public API or not. It appears PlaceIt.net has had an API at some point: https://www.programmableweb.com/api/placeit-rest-api But all the endpoints seem dead so you’d have to investigate that part more first.

Assuming you find a flyer builder with an API, you could then GET and POST the data you need. Integrating it would probably depend on the specific API design they’re offering, and how you would want to work with it. Another question for the future!