This is Finley from the Social Care team at Shopify.
I appreciate you reaching out and understand that you'd like to know what this fee is for.
You can view the details of your Shopify payments payouts at any time by going to your Shopify Admin > Settings > Payments > Shopify Payments > View Payouts like so:
Once there, you can view the details of a specific payout by clicking the amount of the payout, or in the All payouts section, you can click the date of the payout.
If you're in the United States, your credit card rate would be 2.9% + 30¢ USD, which adds up with the screenshot provided.
Though we don't reimburse the credit cate fee for your refunded order, we won’t charge you an additional fee to process the refund, which is common with other payment processors.
I do hope this clarifies things for you. If I can help with anything else, please do let me know.
All the best,