I've fallen a foul of this, out of interest...
The item is your property, if they acknowledge they have it and refuse to return it unless you pay them. What's the legality of that, presumably if they refuse to return it to you... It's theft or larceny by finding and if they still refuse to hand it over at your request, it potentially becomes handling of stolen property, extortion by unlawfully withholding property, dealing in knowingly stolen goods if they subsequently dispose of it.
I'm sure their claim is that it is an 'administration' fee, which may be acceptable if they provide you with a service by sending it to you. But if you collect it from their place of business it seems they would have no legal basis to force you to pay??
However, the operator of the service will be incurring costs, for instance in employing several members of staff and renting a premises to keep the items. Isn't it reasonable that the people responsible for losing their property collectively pay these costs?