In the UK you can use a tax refund app called Airvat
https://airvat.com/ that charges a flat 20% fee. The only limitation is that for receipts above £250 you would need to ask for a full invoice from the retailer so make sure to do this before making the purchase as they would be more helpful. In some larger department stores you would need to email the customer services desk with a request for an invoice so make sure to leave a few days as sometime they can take up to a week to respond. So basically it seems you either have to do some extra work yourself to save on the commission, or do it immediately in the shop with global blue but give up half of the refund.