It's not supposed to be a 'free service'? It is a 'free service'. Just ask for a receipt. I first found this out 15 years ago at Korat IO. I asked the officer, Khun Arcade, after he'd finished transferring the stamps, how much for him, he replied 'no charge, free service.
I did my second transfer a couple of years back at Buriram IO and I was asked for 400 Baht. I said 'no I'm not paying, it's a free service'. Office was full with other farang and I was told to sit back down. I did, but no sooner than I did I got called up and the transfer was done. No charge.