"Invoice Numbers," are optional but if you do not specify, your payment will be applied to the oldest open invoice first.
Once you enter a valid amount, you may select "Pay with credit card" or "Pay with PayPal."
When paying with a credit card, once you have correctly completed all the fields, a "Send Payment" icon will appear. Simply click it to complete transaction.
When paying with PayPal, you will be required to log into your PayPal account to complete the transaction.
When the transaction is complete, you will see the results and the system will email your receipt to the email address provided.