Why did the required payment in payment form not happen?

I've set up an event registration form, that when people choose to pay by credit card, they're required to pay the $250 registration fee. One registrant, however, inadvertently bypassed that required fee, and only paid an optional meal fee. He writes, "When I entered the zip code on the credit card, it clicked off and said “Thank you” for registering. Perhaps I hit an enter button but it happened so fast I don’t remember. Could have been operator error."

Can you look into this to see what might have happened? It hasn't happened to anyone else yet, but with an anticipated 400 or so registrations, we want to ensure this doesn't cause anymore issues. 

We would like to apologize for any inconvenience. I have created a clone version of your form and made a test submission but I wasn't able to reproduce the issue. It could be a temporary glitch. Is the form embedded on a website? Or, it can only be accessed from the same link you shared with us?

I suggest that you observe your form and let us know immediately should this happen again.

You can also try refunding whatever amount is only charged to your registrant and ask him to try resubmitting the form again.