Thanks for trying again Jon! Perhaps the last changes took a little time to be applied.
I've just asked my brother over the phone to try it out:
Since there's two way to pay, (via a paypal account or with a credit card # only) he tried it both via his paypal account, and again, in incognito mode without a paypal account by using only a credit card number. It worked in both instances.
I cannot thank you enough for persisting in trying to make that paypal button work! Let me know if you try again & still have an error issue! Greatly appreciated!
Félix