Free webservice to test if your IBAN number is a valid one. Easy to use.
Test IBAN numbers
Webhook.site to test posting
Use this free website to test if your http/ https posts works, and what data is sent.
I needed to check what data was being posted using some Javascript I found. And I wanted to test this without programming a receiving back-end before. This article on BannerBear explains webhooks and how to use the webhook.site.