After you transition to Stax to power your Shelterpay account, a partial refund (i.e. a refund of only some of the items on a transaction and not all items on a transaction) will only be available to use after 11pm ET the same day that the original transaction was processed. This is because all credit card transactions are settled at 11pm ET by Stax, and they don't allow partial refunds before that time. However, you can still void or fully refund the entire transaction at any point after the original transaction was processed.
If you do refund an entire transaction before 11pm ET on the same day that the original transaction was processed, there will be no credit card processing fee for that original transaction.