Partial refund for customer

I have invoiced a customer using AWS metering, but there is a mistake on my end and I have to initiate a partial refund. I have filled out the refund form after adding the customer ID, and product ID and selecting partial refund, and submitted the form.

Context: The customer has received the invoice from AWS and has not made payment yet to that invoice. They are asking

1. whether they have to make payment of the invoice first
2. when will the refund happen
3. where will they be able to see the refund?