Tie the refund to its purchase
Find the receipt and the original settlement before deciding what to send. Note whether the original amount included a personal item or whether all of it was shared. A refund for a personal accessory inside a shared order should go to the person who paid for that accessory, even if the other adult's card receives it.
Recalculate both final shares
In the kettle example, the household ultimately spent $48. Each adult should finish at $24. The person whose card received $12 has effectively paid $18 after their original $30 contribution. Sending $6 to the other buyer brings both to $24 without changing the original record.
Track what kind of value came back
Mark the refund as expected until the money actually arrives. If the shop sends store credit instead, write down who holds it and what the household wants to use it for. A credit that only one account can spend needs a different plan from cash received.
If one adult keeps the entire store credit for a future personal order, they could settle the other adult's share now. If you both plan to spend it on another household purchase, record who holds the credit and the amount left. Do not also subtract a cash refund that never arrived.