Blunt on purpose: credits are usage units, and once they are used the money has been spent on compute. Here is exactly when a purchase is refundable and how to ask.
All payments — and therefore all refunds — are handled by Paddle.com as Merchant of Record. Approved refunds are issued by Paddle to your original payment method; how fast the money shows up depends on your bank.
Refund requests are accepted within 14 days of the charge. After 14 days a purchase is final.
A credit pack is refundable only if no credits were spent after the purchase. Running any generation after buying — even one — makes the pack non-refundable. There are no partial refunds for partially used packs.
An approved refund (or chargeback) removes the purchased credits from your account. If you already spent some of them, your balance can go negative, and you will not be able to start new generations until it is positive again.
Either way works, and both go to the same place:
Related reading: Terms of Service, section "Credits".