GarageSale loads these fee details from the eBay system. Sometimes nothing is provided by the eBay system (or it just takes a while) and so there is nothing for GarageSale to show.
It’s worth a try to refresh your eBay access token in the GarageSale preferences > Accounts and afterwards select “Update All Listings” from the “Listing” menu and also “Update All Orders” from the “Orders” menu.