Online Payment Form Fails Unless A Field Is Clicked

PHP & JS web form takes customer info then presents a credit card payment window, which will successfully charge the customer - but ONLY IF the customer clicks on a field. They can leave the field empty, they just need to click on it. If they don't, the form will fail with the error message "Unknown error occurred. Please try again later." This is not supposed to happen. This is easily reproduced. Navigate to: [login to view URL] Enter _only_ "Personal Details" (you can use fake info) Don't click on any of the "Car Ownership Info" fields. Click the <Register> button at the bottom and observe. Now, repeat the process as the following: Refresh the page. Enter "Personal Details" again. This time, only click on a "Year" drop-down field, but don't select any value - leave all the "Car Ownership Info" fields blank. Notice the <Register> button changes to <Pay with Card>. Only now it will succeed. This form should work without clicking on a "Car Ownership Info" field.