If the item has been listed as a postage only item, and now the buyer is wishing to collect they will not have that essential QR code for you, this shows collection and you cannot mark it collected any other way. It could lead to the buyer later opening a case for item not received, you would have no proof otherwise and would have to refund.
If they you are happy for them to collect, then message them you'll need to cancel the order which will refund them and relist as a Collection item only. This will release this 6 digit QR code for them, remind them they Must bring this with them, the item will not be released without it.
You then cancel the order citing Buyer requests to cancel, this refunds them and doesn't have any negative impact on your account.
One tip when messaging the buyer make it very , very clear you are cancelling to relist it again on eBay for them to buy from that listing, eBay read all messages, leave no room for doubt that this is Not an off eBay sale.
@mkclothes