I want to know what happens with my payments when a full refund is processed
Refunds are applied to your fourth and final installment payment first, then to your third, and so on until the full refund amount has been credited/issued back to you.
Full Refunds
In the event that you return your entire order and the merchant processes a full refund, your open installment payments will be cancelled, and any installment payments that you have made to date will be refunded to the payment method that was used for the order. This will happen within 13 days of us receiving the funds from the merchant (not 13 days from when you receive a refund email).