The page where I am having the issue is: crcsite.org/payments-rest-of-the-world/ – the site is currently in test mode, so please use one of Stripe’s test cards when testing https://stripe.com/docs/testing#cards. We do not use custom fields, so I doubt it is that. If you also want to see WP Simple Pay working on our site then I have a test page at crcsite.org/aamtestsimplepay/
I am based in the UK hence why I did not reply earlier.