Secure checkout by Stripe. The amount is your current balance, worked out from your invoice, so there's nothing to type. You get a receipt by email, and the payment appears on your booking the moment it goes through.
The quickest way is the Pay online button in your confirmation email. No email handy? Enter your invoice number and booking email:
The card processing fee (Stripe's cost, about 3%) is included in the amount shown at checkout. Added upgrades or extra time later? The same link charges just the new balance.