Yes, of course you can pay your KartHost invoice early. Even if you have a card on file for automated payment.
For details on how to actually pay your invoice see "Paying My Invoice at KartHost"
Since all invoices are paid 1 day before the due date, (to allow our clients plenty of time to change cards should there be a problem) So you can pay your outstanding invoice at any time after its been created until its automatically paid.
What Will Not Happen -
If you pay your invoice early, your invoice will not automatically pay again. Once the invoice is paid, all payments to that specific invoice are completed.
If your invoice is for a Domain Name renewal, you can pay the invoice at any time. When the domain name is renewed, the renewal has no effect on the expiration month and day. Only the year increases by the number of years the domain name was renewed. So if your domain name expires on the 28th of the month, and you pay on the 1st of the month, your domain name will still have a future expiration date of the 28th of the same month.
For details on how to pay your invoice early manually see "Paying My Invoice at KartHost"
updated November 27, 2019