Hi Mike,
Thank you for your purchase.

This is input is Stripe.js library and that library handle all related to this input. I think when you enter the card number from UK, it will recognize that card is from UK and it will remove ZIP code. This is the quote from their documentation:

"Note that ZIP and postal code validation depends on your customer’s billing country. Use our international test cards to experiment with other postal code formats"

So when I add card number from my country it doesn't require ZIP code to be added.

Can you please check again your Test Publishable Key and Test Secret Key on your Stripe account? 

With Stripe or PayPal there is only two ways where customers can made mistakes. First one is to use Live credentials when Test/Sandbox mode is enabled, and second one it to copy wrong Stripe/PayPal credentials.  

If you insert right data and still not working, please provide me a temporary WP-admin login for your site where this happens, so we could log in and take a look ‘from the inside’ as that’s the most efficient way to see and resolve the issue. We do not interfere with any data or anything else except for the plugin (in case that’s a production version of the site), and of course we do not provide login data to third party. You can write credentials here just check Private Reply so nobody can see them except us.

Best regards.