You can always make additional payments by making a payment using your debit card in the Zable App.

Once you have a completed transaction on your account you will see the option to "make a one-off payment" in the repay section of the app.

Please note that you wouldn't be able to make repayments towards a pending payment, however, once it has completed you can do so in the repay section as stated above. Interest is not charged on pending transactions.
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