If you head to your order history page www.skiddle.com/orders you'll be able to pay off your remaining balance from there. 

This can be paid with the card you originally made the order with or any other, it really doesn't matter. 

If you're struggling to log in then you can reset your password by heading to https://www.skiddle.com/register/forgottenpassword.php

If this reset confirmation doesn't appear in your inbox, please ensure to double check your junk / spam folder