I bought a phone and they told me my payment was flagged as "suspicious" so they debited a small amount to prove it was me. I had to waste time emailing backwards and forwards and phoning my bank to find out the amount and let them know. Then they approved my transaction. The amount was only 18 cents which was debited along with the full amount for my phone. The 18 cents was never credited back. I know this is only a tiny amount and the reason that I couldn't be bothered going through the hassle of trying to claim it back but I suspect they do this to every customer. Firstly they are withdrawing an amount they were never authorised to withdraw and then they never give it back. It's another way of ripping off customers.
I'd still complain to them, even though they almost certainly won't do anything. Also might be worthwhile sending a quick message to your bank to let them know about it, without actually filing a chargeback.